Merge pull request swiftlang#34259 from apple/shahmishal/fix-getting-started-doc

Update GettingStarted doc to use `--scheme` for update-checkout script

Author: shahmishal

docs/HowToGuides/GettingStarted.md

@@ -97,7 +97,7 @@ toolchain as a one-off, there are a couple of differences:
    or a specific snapshot. You can update the branch/tag for all repositories
    as follows:
    ```sh
-   utils/update-checkout --branch mybranchname
+   utils/update-checkout --scheme mybranchname
    # OR
    utils/update-checkout --tag mytagname
    ```