doc: update issue reference format in CONTRIBUTING.md (googleapis#13382)

Fix the last line of the commit message example, which previously had an
unnecessary slash.

Author: julieqiu

CONTRIBUTING.md

@@ -124,7 +124,7 @@ feat(storage): add new storage bucket feature
 
 A new feature is added to storage.
 
-Fixes \#238
+Fixes #238
 ```
 
 ### First line