(MAINT) Fix AvoidUsingDoubleQuotesForConstantString in overview README (#1883)

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

Author: michaeltlombardi

Diff for: docs/Rules/README.md

@@ -31,7 +31,7 @@ The PSScriptAnalyzer contains the following rule definitions.
 | [AvoidUsingComputerNameHardcoded](./AvoidUsingComputerNameHardcoded.md)                           | Error       |        Yes         |                 |
 | [AvoidUsingConvertToSecureStringWithPlainText](./AvoidUsingConvertToSecureStringWithPlainText.md) | Error       |        Yes         |                 |
 | [AvoidUsingDeprecatedManifestFields](./AvoidUsingDeprecatedManifestFields.md)                     | Warning     |        Yes         |                 |
-| [AvoidUsingDoubleQuotesForConstantString](./AvoidUsingDoubleQuotesForConstantString.md)           | Warning     |         No         |       Yes       |
+| [AvoidUsingDoubleQuotesForConstantString](./AvoidUsingDoubleQuotesForConstantString.md)           | Information |         No         |                 |
 | [AvoidUsingEmptyCatchBlock](./AvoidUsingEmptyCatchBlock.md)                                       | Warning     |        Yes         |                 |
 | [AvoidUsingInvokeExpression](./AvoidUsingInvokeExpression.md)                                     | Warning     |        Yes         |                 |
 | [AvoidUsingPlainTextForPassword](./AvoidUsingPlainTextForPassword.md)                             | Warning     |        Yes         |                 |