Specify '--request PUT' in cURL call example

doc/api/merge_requests.md

@@ -1632,7 +1632,7 @@ PUT /projects/:id/merge_requests/:merge_request_iid/rebase
 | `skip_ci`           | boolean | no       | Set to `true` to skip creating a CI pipeline |
 
 ```shell
-curl --header "PRIVATE-TOKEN: <your_access_token>" https://gitlab.example.com/api/v4/projects/76/merge_requests/1/rebase
+curl --request PUT --header "PRIVATE-TOKEN: <your_access_token>" https://gitlab.example.com/api/v4/projects/76/merge_requests/1/rebase
 ```
 
 This is an asynchronous request. The API will return a `202 Accepted` response