docs(readme): fix code indent (#401)

Author: Julien Bouquillon

README.md

@@ -161,9 +161,9 @@ You can set it up like this:
 
 ```yaml
 - uses: actions/checkout@v2
-    with:
-      repository: ${{ github.event.pull_request.head.repo.full_name }}
-      ref: ${{ github.event.pull_request.head.ref }}
+  with:
+    repository: ${{ github.event.pull_request.head.repo.full_name }}
+    ref: ${{ github.event.pull_request.head.ref }}
 ```
 
 You can find the full docs for payloads of `pull_request` events [here](https://docs.github.com/en/developers/webhooks-and-events/webhooks/webhook-events-and-payloads#webhook-payload-example-32).