Fix typo in Readme (Window Server -> Windows Server)

[erenes]

PR Summary

Fix a typo in the word "Windows" 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 Dockerfile, .sh, .ps1 and .psm1 files have the correct copyright header
• 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.
• Adds a new image or tag
  • Not Applicable
• OR
  • Update README.powershellcommunity.md
  • Update vsts-ci.yml