ci: fix spelling action failure (a2aproject#926)

See a2aproject#922, `pull_request_target` apparently requires pushing to `main`.

Author: ishymko

.github/workflows/spelling.yaml

@@ -1,7 +1,7 @@
 ---
 name: Check Spelling
 on:
-  pull_request:
+  pull_request_target:
     branches: ['**']
     types: [opened, reopened, synchronize]
   issue_comment:
@@ -11,6 +11,7 @@ jobs:
     name: Check Spelling
     permissions:
       contents: read
+      pull-requests: read
       actions: read
       security-events: write
     outputs: