ci: fix spelling action failure (#922)

There is [code in spelling
check](https://github.com/check-spelling/check-spelling/blob/c635c2f3f714eec2fcf27b643a1919b9a811ef2e/unknown-words.sh#L182)
which probes branch renaming to make sure that pipeline is set up
according to the
[guide](https://docs.check-spelling.dev/Feature:-Restricted-Permissions).

It doesn't specify request body and looks for a specific message in the
response assuming that token check happens before body validation. Seems
like GitHub change behavior and body schema is validated before
permissions, so such request fails with a different error now:

```
curl -s -X POST \
  -H "Accept: application/vnd.github+json" \
  -H "Authorization: Bearer $GITHUB_TOKEN" \
  "https://api.github.com/repos/a2aproject/a2a-python/branches/1.0-dev/rename"

{
  "message": "Invalid request.\n\nFor 'links/0/schema', nil is not an object.",
  "documentation_url": "https://docs.github.com/rest/branches/branches#rename-a-branch",
  "status": "422"
}
```

Providing body gives a proper error:

```json
{
  "message": "Resource not accessible by personal access token",
  "documentation_url": "https://docs.github.com/rest/branches/branches#rename-a-branch",
  "status": "403"
}
```

Trying to update the workflow according to the sample
https://raw.githubusercontent.com/check-spelling/spell-check-this/main/.github/workflows/spelling.yml.

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: