agency: Vehicle Update response code should be 200

According to the RFC : https://tools.ietf.org/html/rfc7231#section-6.3.1
the HTTP response for an update (that does not create any object) can
either be 200 (OK) or 202 (ACCEPTED)
The later "indicates that the request has been
accepted for processing, but the processing has not been completed."

Since updating the vehicle_id should be quite fast, we should use a 200
response code.

Author: Antoine LAURENT

agency/README.md

@@ -139,7 +139,7 @@ Body Params:
 | ------------ | ------- | ----------------- | -------------------------------------------------------------------- |
 | `vehicle_id` | String  | Required          | Vehicle Identification Number (vehicle_id) visible on vehicle               |
 
-201 Success Response:
+200 Success Response:
 
 _No content returned on success._