Fix name of PowerShell 5.1 test stage in CI

PR Summary

As per title.

[x] PR has a meaningful title
- Use the present tense and imperative mood when describing your changes
[x] Summarized changes
[x] Change is not breaking
[x] Make sure all .cs, .ps1 and .psm1 files have the correct copyright header
[ ] Make sure you've added a new test if existing tests do not effectively test the code changed and/or updated documentation
[x] This PR is ready to merge and is not Work in Progress.
- If the PR is work in progress, please add the prefix WIP: to the beginning of the title and remove the prefix when the PR is ready.