Fix docs after namespacing Ci::ExternalPullRequest
What does this MR do and why?
Fix docs after namespacing Ci::ExternalPullRequest
In !122872 (merged)
the ExternalPullRequest
model was renamed to Ci::ExternalPullRequest
but the related file db/docs/external_pull_requests.yml
was not updated.
This change updates the docs with the new class name.
Related to #273605 (closed)