docs: fix markdown linting error in CONTRIBUTING.md

Add 'text' language tag to code fence block to satisfy markdownlint requirements.

Co-authored-by: Damien Storm <[email protected]>

Author: github-actions[bot]

CONTRIBUTING.md

@@ -114,7 +114,7 @@ Semantic versioning and releases are automated in CI (e.g., `python-semantic-rel
 
 PR titles are validated automatically and must follow this format:
 
-```
+```text
 <type>(<optional scope>): <description>
 ```