Fix the action of a room key request cancellation.

The spec states that the action of a room key request cancellation
should be "cancel_request" but every known implementation uses
"request_cancellation" instead.

This patch fixes the spec to reflect the implementations.

event-schemas/examples/m.room_key_request$cancel_request

@@ -1,6 +1,6 @@
 {
   "content": {
-    "action": "cancel_request",
+    "action": "request_cancellation",
     "requesting_device_id": "RJYKSTBOIE",
     "request_id": "1495474790150.19"
   },

event-schemas/schema/m.room_key_request

@@ -38,7 +38,7 @@ properties:
       action:
         enum:
           - request
-          - cancel_request
+          - request_cancellation
         type: string
       requesting_device_id:
         description: ID of the device requesting the key.