docs: update GH Action snippet: on.pull_request conditions (#28)

## 🧰 Changes

Sync latest changes from the GH action snippet in the Micro webapp.

## 🧬 QA & Testing

- This workflow has already been tested to work.
- Confirm this snippet matches the one used during Micro onboarding.

README.md

@@ -16,17 +16,17 @@ name: ReadMe Micro
 # Run workflow to sync OpenAPI files for every push to the `main` branch
 on:
   push:
-    branches:
-      - main
+    branches: [main]
   pull_request:
-    types: [opened, synchronize]
+    branches: [main]
 jobs:
   sync:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout this repo
         uses: actions/checkout@v3
 
+      # Run GitHub Action to sync all OpenAPI files in the repo
       - name: GitHub Action
         uses: readmeio/[email protected]
         with: