fix an example in authentication-in-a-workflow

actions/labeler now recommends to use the `pull_request_target` event.
With the `pull_request` event, `GITHUB_TOKEN` sometimes lacks write permissions (e.g. pull requests from forked repository or from dependabot)
and actions/labeler doesn't work well.

ref. actions/labeler#90
ref. https://github.blog/2020-08-03-github-actions-improvements-for-fork-and-pull-request-workflows/#improvements-for-public-repository-forks

Author: shogo82148

content/actions/reference/authentication-in-a-workflow.md

@@ -40,7 +40,7 @@ This example workflow uses the [labeler action](https://github.com/actions/label
   ```yaml
   name: Pull request labeler
   on:
-  - pull_request
+  - pull_request_target
   jobs:
     triage:
       runs-on: ubuntu-latest