typo fix

Author: boomanaiden154

llvm/utils/git/code-format-helper.py

@@ -94,7 +94,7 @@ def pr_comment_text_for_diff(self, diff: str) -> str:
 
 :warning:
 The reproduction instructions above might return results for more than one PR
-a stack if you are using a stacked PR workflow. You can limit the results by
+in a stack if you are using a stacked PR workflow. You can limit the results by
 changing `origin/main` to the base branch/commit you want to compare against.
 :warning: