Fix callback key expression example.

Author: Jason Walton

versions/3.0.2.md

@@ -1883,7 +1883,7 @@ $method | POST
 $request.path.eventType | myevent
 $request.query.queryUrl | http://clientdomain.com/stillrunning
 $request.header.content-Type | application/json
-$request.body#/failedUrl | http://clientdomain.com/stillrunning
+$request.body#/failedUrl | http://clientdomain.com/failed
 $request.body#/successUrls/2 | http://clientdomain.com/medium
 $response.header.Location | http://example.org/subscription/1