Add import in docs. [ci skip]

Author: tomchristie

Diff for: docs/api-guide/throttling.md

@@ -184,6 +184,8 @@ If the `.wait()` method is implemented and the request is throttled, then a `Ret
 
 The following is an example of a rate throttle, that will randomly throttle 1 in every 10 requests.
 
+    import random
+
     class RandomRateThrottle(throttling.BaseThrottle):
         def allow_request(self, request, view):
             return random.randint(1, 10) == 1