Don't catch all exceptions in Directory.CreateDirectory example

[adamsitnik]

Summary

The CreateDirectory can fail with few exceptions:

• ArgumentException, ArgumentNullException, PathTooLongException, DirectoryNotFoundException or NotSupportedException when the path is invalid
• UnauthorizedAccessException when the caller does not have the required permission.

Since this example uses a harcoded, short and valid path c:\MyDir there is no need to catch any other exceptions than UnauthorizedAccessException

Fixes #11240

[BillWagner]

Thanks @adamsitnik

This looks great, and I'll now.