docs: PRマージ時にストア公開のワークフローが発火するように変更

.github/workflows/submit.yml

@@ -1,11 +1,13 @@
 name: "Submit to Web Store"
 on:
   workflow_dispatch:
-  release:
-    types: [published]
+  pull_request:
+    types:
+      - closed
 
 jobs:
   build:
+    if: github.event_name == 'pull_request' && github.event.pull_request.merged == true
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v3