Fix `about_Windows_Powershell_5.1.md` capitalization

[surfingoldelephant]

Prerequisites

• Existing Issue: Search the existing issues for this repository. If there is an issue that fits your needs do not file a new one. Subscribe, react, or comment on that issue instead.
• Descriptive Title: Write the title for this issue as a short synopsis. If possible, provide context. For example, "Typo in Get-Foo cmdlet" instead of "Typo."
• Verify Version: If there is a mismatch between documentation and the behavior on your system, ensure that the version you are using is the same as the documentation. Check this box if they match or the issue you are reporting is not version specific.

Links

https://learn.microsoft.com/en-us/powershell/module/microsoft.powershell.core/about/about_windows_powershell_5.1
https://learn.microsoft.com/en-us/powershell/module/microsoft.powershell.core/about/about
https://learn.microsoft.com/en-us/powershell/scripting/learn/ps101/02-help-system

Summary

The about_Windows_Powershell_5.1.md file name doesn't use the correct PowerShell capitalization and should be changed to about_Windows_PowerShell_5.1.md.

Per the contributor's guide, I'm submitting an issue rather than just addressing this in a PR as it involves renaming a file.

Suggested Fix

about_Windows_Powershell_5.1.md:

• Rename the file to about_Windows_PowerShell_5.1.md (capitalize the S).
• Change Line 7 to title: about_Windows_PowerShell_5.1.

About.md:

• Change Line 630 to ### [about_Windows_PowerShell_5.1](about_Windows_PowerShell_5.1.md).

02-help-system.md:

• Change Line 600 to about_Windows_PowerShell_5.1 HelpFile