docs: fix cleanup example to use pull_request_target trigger

[Mogyuchi]

Description

Change the workflow trigger event from pull_request to pull_request_target in the "cleanup caches by a branch" workflow example.

Motivation and Context

When triggered by the pull_request event, the GITHUB_TOKEN does not have permission to delete the cache of the base repository in a cross-repository pull request. This results in the error Error: Resource not accessible by integration.
This problem can be avoided by using the pull_request_target event.

How Has This Been Tested?

I've updated the workflow to use the pull_request_target event and triggered a GitHub Actions run.

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (add or update README or docs)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.