fix merge_request_author_approval comment

It have verified on gitlab.com that logic for
merge_request_author_approval is working this way:
* true - MR author can approve
* false - MR author cannot approve

doc/api/merge_request_approvals.md

@@ -51,7 +51,7 @@ POST /projects/:id/approvals
 | `approvals_before_merge`                         | integer | no       | How many approvals are required before an MR can be merged. Deprecated in 12.0 in favor of Approval Rules API. |
 | `reset_approvals_on_push`                        | boolean | no       | Reset approvals on a new push                                                                       |
 | `disable_overriding_approvers_per_merge_request` | boolean | no       | Allow/Disallow overriding approvers per MR                                                          |
-| `merge_requests_author_approval`                 | boolean | no       | Allow/Disallow authors from self approving merge requests; `true` means authors cannot self approve |
+| `merge_requests_author_approval`                 | boolean | no       | Allow/Disallow authors from self approving merge requests; `true` means authors can self approve |
 | `merge_requests_disable_committers_approval`     | boolean | no       | Allow/Disallow committers from self approving merge requests                                        |
 | `require_password_to_approve`                    | boolean | no       | Require approver to enter a password in order to authenticate before adding the approval         |