added target_project_id to merge_requests.md

Added description about optional field target_project_id for merge requests.

doc/api/merge_requests.md

@@ -109,6 +109,7 @@ Parameters:
 + `target_branch` (required)     - The target branch
 + `assignee_id` (optional)       - Assignee user ID
 + `title` (required)             - Title of MR
++ `target_project_id` (optional) - The target project (numeric id)
 
 ```json
 {