Merge branch 'patch-1' into 'master'

Update merge_requests.md.

Error in 'Approve merge request'  section in API documentation

## What does this MR do?

Just fix an error in API Documentation in 'Approve Merge Request' section. 

Url in documentation : 
`POST /projects/:id/merge_requests/:merge_request_id/approvals`

Should be : 
`POST /projects/:id/merge_requests/:merge_request_id/approve`

(Verified on gitlab 8.12.7-ee 4051516)

See merge request !809

doc/api/merge_requests.md

@@ -588,7 +588,7 @@ If you are allowed to, you can approve a merge request using the following
 endpoint:
 
 ```
-POST /projects/:id/merge_requests/:merge_request_id/approvals
+POST /projects/:id/merge_requests/:merge_request_id/approve
 ```
 
 **Parameters:**