Update how-to-set-up-docs-previews.md

Author: reakaleek

docs/migration/guide/how-to-set-up-docs-previews.md

@@ -26,7 +26,7 @@ name: docs-build <1>
 
 on:
   push: <2>
-    - branches: 
+    branches: 
         - main 
   pull_request: ~