ReadMe: Remove reference to Windows PowerShell in Introduction

[hjorslev]

PR Summary

With PowerShell being cross platform I suggest that the two references to Windows PowerShell be changed to just PowerShell in the introduction in the readme.

PR Checklist

• PR has a meaningful title
  • Use the present tense and imperative mood when describing your changes
• Summarized changes
• Change is not breaking
• Make sure all .cs, .ps1 and .psm1 files have the correct copyright header
• NA Make sure you've added a new test if existing tests do not effectively test the code changed and/or updated documentation
• 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.

[bergmeister]

Thanks