Update header capitalization in contributing.md

Author: Daniel Abraham

docs/contributing.md

@@ -15,7 +15,7 @@ You generally only need to submit a CLA once, so if you've already submitted one
 (even if it was for a different project), you probably don't need to do it
 again.
 
-## Code reviews
+## Code Reviews
 
 All submissions, including submissions by project members, require review. We
 use GitHub pull requests for this purpose. Consult