Updated curl command's data parameter to create a new subgroup. It was missing a single quote(').

doc/api/groups.md

@@ -696,7 +696,7 @@ This is similar to creating a [New group](#new-group). You'll need the `parent_i
 
 ```shell
 curl --request POST --header "PRIVATE-TOKEN: <your_access_token>" --header "Content-Type: application/json" \
-  --data '{"path": "<subgroup_path>", "name": "<subgroup_name>", "parent_id": <parent_group_id> } \
+  --data '{"path": "<subgroup_path>", "name": "<subgroup_name>", "parent_id": <parent_group_id> }' \
   "https://gitlab.example.com/api/v4/groups/"
 ```