Update plain_rules.md

docs/API/plain_rules.md

@@ -404,7 +404,7 @@ the Perseo configuration). The `parameters` map includes the following fields:
 -   authentication: optional, authentication (host, port, user, password and service) configuration values that will be
     used by updateAction rule (instead of default authentication defined by configuration) which will be used when a
     trust token should be negotiated. i.e.:
-    ```
+    ```json
     "authentication": {
         "host": "ext-keystone",
         "port": 5001,