Update CF V3 API definition (#4458)

In all the example request sections the "curl" command contains explicit "-X" parameter
In this endpoint it does not and it is kind of inconsistent with the rest of the documentation

Author: rrashidov

docs/v3/source/includes/resources/routes/_list_shared_spaces.md.erb

@@ -8,6 +8,7 @@ Example Request
 
 ```shell
 curl "https://api.example.org/v3/routes/[guid]/relationships/shared_spaces" \
+  -X GET \
   -H "Authorization: bearer [token]" \
   -H "Content-type: application/json"
 ```