ref(API): rc got renamed to exit_code in the API

Required for deis/controller#738

Author: helgi

src/reference-guide/controller-api/v2.0.md

@@ -359,7 +359,7 @@ DEIS_API_VERSION: 2.0
 DEIS_PLATFORM_VERSION: 2.0.0
 Content-Type: application/json
 
-{"rc": 0, "output": "hi\n"}
+{"exit_code": 0, "output": "hi\n"}
 ```
 
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