Fixes method in docs for MergeRequests#cancel_merge_when_pipeline_succeeds API call

The actual API uses the POST method instead of PUT: https://gitlab.com/gitlab-org/gitlab/blob/bf24aef3d8a3b422c56f4d6f29914708b7ceb38d/lib/api/merge_requests.rb#L447.

doc/api/merge_requests.md

@@ -1407,7 +1407,7 @@ If the merge request is already merged or closed - you get `405` and error messa
 In case the merge request is not set to be merged when the pipeline succeeds, you'll also get a `406` error.
 
 ```
-PUT /projects/:id/merge_requests/:merge_request_iid/cancel_merge_when_pipeline_succeeds
+POST /projects/:id/merge_requests/:merge_request_iid/cancel_merge_when_pipeline_succeeds
 ```
 
 Parameters: