Fix in labeler.yml (microsoft#1822)

### Motivation and Context
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Labeler actions on PRs are currently failing due to a broken match rule.

### Description
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Add required quotation marks for match rule with leading asterisk (see
[reference](https://github.com/actions/labeler#basic-examples)).

.github/labeler.yml

@@ -36,4 +36,4 @@ copilot chat:
 # Add 'documentation' label to any change within the 'docs' directory, or any '.md' files
 documentation:
   - docs/**/*
-  - **/*.md
+  - '**/*.md'