Fix links in us_ticker.md

Fix links for proper rendering on test site.

Author: Amanda Butler

docs/reference/contributing/target/us_ticker.md

@@ -42,7 +42,7 @@ To implement this API, the device must have a hardware counter that has a count
 
 You can find the API and specification for the microsecond ticker API in the following header file:
 
-[![View code](https://www.mbed.com/embed/?type=library)](https://os-doc-builder.test.mbed.com/docs/development/mbed-os-api-doxy/group__hal__us__ticker.html)
+[![View code](https://www.mbed.com/embed/?type=library)](http://os-doc-builder.test.mbed.com/docs/development/mbed-os-api-doxy/group__hal__us__ticker.html)
 
 To enable microsecond ticker support in Mbed OS, add the `USTICKER` label in the `device_has` option of the target's section in the `targets.json` file.
 
@@ -56,8 +56,8 @@ mbed test -t <toolchain> -m <target> -n tests-mbed_hal-common_ticker*,tests-mbed
 
 You can read more about the test cases:
 
- [![View code](https://www.mbed.com/embed/?type=library)](https://os-doc-builder.test.mbed.com/docs/development/mbed-os-api-doxy/group__hal__us__ticker.html)
+ [![View code](https://www.mbed.com/embed/?type=library)](http://os-doc-builder.test.mbed.com/docs/development/mbed-os-api-doxy/group__hal__us__ticker.html)
 
- [![View code](https://www.mbed.com/embed/?type=library)](https://os-doc-builder.test.mbed.com/docs/development/mbed-os-api-doxy/group__hal__ticker__tests.html)
+ [![View code](https://www.mbed.com/embed/?type=library)](http://os-doc-builder.test.mbed.com/docs/development/mbed-os-api-doxy/group__hal__ticker__tests.html)
 
- [![View code](https://www.mbed.com/embed/?type=library)](https://os-doc-builder.test.mbed.com/docs/development/mbed-os-api-doxy/group__hal__us__ticker__tests.html)
+ [![View code](https://www.mbed.com/embed/?type=library)](http://os-doc-builder.test.mbed.com/docs/development/mbed-os-api-doxy/group__hal__us__ticker__tests.html)