fix: change section name to match method being documented (#362)

fix: fixes helptags, removes duplicate entry

This is a #PATCH release

doc/gitlab.nvim.txt

@@ -757,7 +757,7 @@ By default, the endpoint will return all open merge requests.
             • {labels}: (table<string>) Return merge requests with *including* matching labels
             • Etc, see: https://docs.gitlab.com/ee/api/merge_requests.html#list-merge-requests
 
-                                                                *gitlab.nvim.choose_merge_request*
+                                                                *gitlab.nvim.choose_merge_request_by_username*
 gitlab.choose_merge_request_by_username({opts}) ~
 
 Choose a merge request based on the username provided. Like the `choose_merge_request`