Set the link for api perms to user/permissions

doc/api/merge_trains.md

@@ -3,7 +3,7 @@
 > - [Introduced](https://gitlab.com/gitlab-org/gitlab/issues/36146) in GitLab 12.9.
 > - Using this API you can consume GitLab's [Merge Train](../ci/merge_request_pipelines/pipelines_for_merged_results/merge_trains/index.md) entries.
 
-Every API call to merge trains must be authenticated with Developer or higher [permissions](link-to-permissions-doc).
+Every API call to merge trains must be authenticated with Developer or higher [permissions](../user/permissions.md).
 
 If a user is not a member of a project and the project is private, a `GET` request on that project will result to a `404` status code.