Typo fix on compute/oslogin/service_account_ssh.py (#4420)

## Description

Fixes typo on compute/oslogin/service_account_ssh.py. No issue created.

## Checklist
- [X] I have followed [Sample Guidelines from AUTHORING_GUIDE.MD](https://github.com/GoogleCloudPlatform/python-docs-samples/blob/master/AUTHORING_GUIDE.md)
- [X] README is updated to include [all relevant information](https://github.com/GoogleCloudPlatform/python-docs-samples/blob/master/AUTHORING_GUIDE.md#readme-file)
- [X] **Tests** pass:   `nox -s py-3.6` (see [Test Environment Setup](https://github.com/GoogleCloudPlatform/python-docs-samples/blob/master/AUTHORING_GUIDE.md#test-environment-setup))
- [X] **Lint** pass:   `nox -s lint` (see [Test Environment Setup](https://github.com/GoogleCloudPlatform/python-docs-samples/blob/master/AUTHORING_GUIDE.md#test-environment-setup))
- [ ] These samples need a new **API enabled** in testing projects to pass (let us know which ones)
- [ ] These samples need a new/updated **env vars** in testing projects set to pass (let us know which ones)
- [X] Please **merge** this PR for me once it is approved.

Tests were done with `nox -s py-3.7` not `nox -s py-3.6`.

Complete nox runs:
```
rdcr@cloudshell:~/python-docs-samples/compute/oslogin$ /home/rdcr/.local/bin/nox -s lint
No user noxfile_config found: detail: No module named 'noxfile_config'
nox > Running session lint
nox > Creating virtual environment (virtualenv) using python3 in .nox/lint
nox > pip install flake8 flake8-import-order
nox > flake8 --show-source --builtin=gettext --max-complexity=20 --import-order-style=google --exclude=.nox,.cache,env,lib,generated_pb2,*_pb2.py,*_pb2_grpc.py --ignore=E121,E123,E126,E203,E226,E24,E266,E501,E704,
W503,W504,I202 --max-line-length=88 --application-import-names service_account_ssh_test,.nox,noxfile,service_account_ssh .
nox > Session lint was successful.
```

```
rdcr@cloudshell:~/python-docs-samples/compute/oslogin$ /home/rdcr/.local/bin/nox -s py-2.7
No user noxfile_config found: detail: No module named 'noxfile_config'
nox > Running session py-2.7
nox > Creating virtual environment (virtualenv) using python2.7 in .nox/py-2-7
nox > Session py-2.7 skipped: SKIPPED: 2.7 tests are disabled for this sample..
```

```
nox > pip install -r requirements-test.txt
nox > pytest --junitxml=sponge_log.xml
================================================================================================ test session starts ================================================================================================
platform linux -- Python 3.7.3, pytest-5.4.3, py-1.9.0, pluggy-0.13.1 -- /home/rdcr/python-docs-samples/compute/oslogin/.nox/py-3-7/bin/python
cachedir: .pytest_cache
rootdir: /home/rdcr/python-docs-samples, inifile: pytest.ini
collected 1 item                                                                                                                                                                                                    

service_account_ssh_test.py::test_main PASSED                                                                                                                                                                 [100%]

----------------------------------------------------------------- generated xml file: /home/rdcr/python-docs-samples/compute/oslogin/sponge_log.xml -----------------------------------------------------------------
=========================================================================================== 1 passed in 115.10s (0:01:55) ===========================================================================================
nox > Session py-3.7 was successful.
```

Author: rjmco

compute/oslogin/service_account_ssh.py

@@ -153,7 +153,7 @@ def main(cmd, project, instance=None, zone=None,
         help='Your Google Cloud project ID.')
     parser.add_argument(
         '--zone',
-        help='The zone where the target instance is locted.')
+        help='The zone where the target instance is located.')
     parser.add_argument(
         '--instance',
         help='The target instance for the ssh command.')