docs: use GH base sha to lint PR commits (#4198)

Author: robjweiss

docs/guides/ci-setup.md

@@ -46,7 +46,7 @@ jobs:
 
       - name: Validate PR commits with commitlint
         if: github.event_name == 'pull_request'
-        run: npx commitlint --from ${{ github.event.pull_request.head.sha }}~${{ github.event.pull_request.commits }} --to ${{ github.event.pull_request.head.sha }} --verbose
+        run: npx commitlint --from ${{ github.event.pull_request.base.sha }} --to ${{ github.event.pull_request.head.sha }} --verbose
 ```
 
 ## Travis