Fix grammar: change "contain" to "contains" (#12788)

This PR fixes a small grammar issue in the repository structure section.

Changed:
"redir - contain redirection mapping files"
to
"redir - contains redirection mapping files"

Thank you!

Author: putku45

README.md

@@ -24,7 +24,7 @@ The following list describes the main folders in this repository.
 - `.github` - contains configuration settings used by GitHub for this repository
 - `.vscode` - contains configuration settings and recommended extensions for VS Code
 - `assets` - contains downloadable files linked in the documentation
-- `redir` - contain redirection mapping files
+- `redir` - contains redirection mapping files
 - `reference` - contains the documentation published to [learn.microsoft.com][01]. This includes
   both reference and conceptual content.
   - `5.1` - contains the cmdlet reference and about topics for PowerShell 5.1