Update codeql.yml

Author: lauren-mills

.github/workflows/codeql.yml

@@ -22,7 +22,7 @@ jobs:
 
     # If this run was triggered by a pull request event, then checkout
     # the head of the pull request instead of the merge commit.
-    - run: git checkout HEAD^2
+    - run: git checkout
       if: ${{ github.event_name == 'pull_request' }}
 
     # Initializes the CodeQL tools for scanning.