Fix `AvoidUsingDoubleQuotesForConstantString` information in overview README

[michaeltlombardi]

PR Summary

As identified[^1] by @BenedekFarkas, the line in the rules overview readme for AvoidUsingDoubleQuotesForConstantString incorrectly lists the severity as Warning and that the rule is configurable when the severity is Information[^2] and there aren't any configurable options.

This change copies the fix @BenedekFarkas kindly submitted to the docs site to ensure the documentation stays up-to-date.

[^1]: For more information, see:

  - MicrosoftDocs/PowerShell-Docs-Modules#128

[^2]: Source code:

  https://github.com/PowerShell/PSScriptAnalyzer/blob/9a6ce978d870909e688d9da0d101d3fc00b504f0/Rules/AvoidUsingDoubleQuotesForConstantString.cs#L142

PR Checklist

• [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
• [x] 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.