Fix possessives used in Plan documentation

These pages in the Plan docset used the possessive form of GitLab,
which isn't OK for legal reasons.

doc/user/project/issues/index.md

@@ -35,7 +35,7 @@ you can also view all the issues collectively at the group level.
 See also [Always start a discussion with an issue](https://about.gitlab.com/blog/2016/03/03/start-with-an-issue/).
 
 <i class="fa fa-youtube-play youtube" aria-hidden="true"></i>
-To learn how GitLab's Strategic Marketing department uses GitLab issues with [labels](../labels.md) and
+To learn how the GitLab Strategic Marketing department uses GitLab issues with [labels](../labels.md) and
 [issue boards](../issue_board.md), see the video on
 [Managing Commitments with Issues](https://www.youtube.com/watch?v=cuIHNintg1o&t=3).
 

doc/user/project/issues/managing_issues.md

@@ -234,7 +234,7 @@ This translates to the following keywords:
 - Resolve, Resolves, Resolved, Resolving, resolve, resolves, resolved, resolving
 - Implement, Implements, Implemented, Implementing, implement, implements, implemented, implementing
 
-Note that `%{issue_ref}` is a complex regular expression defined inside GitLab's
+Note that `%{issue_ref}` is a complex regular expression defined inside the GitLab
 source code that can match references to:
 
 - A local issue (`#123`).

doc/user/project/quick_actions.md

@@ -13,7 +13,7 @@ info: To determine the technical writer assigned to the Stage/Group associated w
 >   quick actions when updating the description of issues, epics, and merge requests.
 
 Quick actions are textual shortcuts for common actions on issues, epics, merge requests,
-and commits that are usually done by clicking buttons or dropdowns in GitLab's UI.
+and commits that are usually done by clicking buttons or dropdowns in the GitLab UI.
 You can enter these commands in the description or in comments of issues, epics, merge requests, and commits.
 Each command should be on a separate line in order to be properly detected and executed.