Try git diff to verify if there are local changes to the cli reference

Author: cmcgee1024

.github/workflows/pull_request.yml

@@ -69,7 +69,7 @@ jobs:
       - name: Prepare the action
         run: ./scripts/prep-gh-action.sh && ./scripts/install-libarchive.sh
       - name: Generate Swiftly CLI Reference and Check for Differences
-        run: swift package plugin --allow-writing-to-package-directory generate-docs-reference && git config --global --add safe.directory $(pwd) && git diff-index --quiet HEAD --  || (echo "The documentation hasn't been updated with the latest swiftly command-line reference. Please run 'swift package plugin generate-docs-reference' and commit/push the changes."; exit 1)
+        run: swift package plugin --allow-writing-to-package-directory generate-docs-reference && git config --global --add safe.directory $(pwd) && git diff --exit-code Documentation/SwiftlyDocs.docc/swiftly-cli-reference.md || (echo "The documentation hasn't been updated with the latest swiftly command-line reference. Please run 'swift package plugin generate-docs-reference' and commit/push the changes."; exit 1)
       - name: Generate Documentation Set
         run: swift package --allow-writing-to-directory .build/docs generate-documentation --target SwiftlyDocs --output-path .build/docs
       - name: Upload Documentation Artifacts