Fix shell name validation example and test

[fornwall]

The shell name validation check documentation has the following example:

  linux:
    runs-on: ubuntu-latest
    steps:
      - run: echo 'hello'
        # ERROR: Unavailable shell
        shell: dash
      - run: echo 'hello'
        # ERROR: 'powershell' is only available on Windows
        shell: powershell
      - run: echo 'hello'
        # OK: 'powershell' is only available on Windows
        shell: powershell

As done in this PR, the OK: 'powershell' is only available on Windows section should be moved to the Windows section.

[rhysd]

Ah, thanks for catching this!