Test against PR ref not HEAD

Author: CRThaze

.github/workflows/preview-readme.yaml

@@ -25,7 +25,7 @@ jobs:
       - name: Check for README changes
         id: check-changes
         run: |
-          if git diff --quiet HEAD -- README.md
+          if git diff --quiet ${{ github.event.pull_request.head.ref }} -- README.md
           then
             echo "has_changes=false" >> $GITHUB_OUTPUT
           else