Fix Rubocop offense

```
Offenses:

activesupport/lib/active_support/subscriber.rb:91:17:
C: Layout/SpaceAroundOperators: Operator = should be surrounded by a single space.
      event     = event_stack.pop
```

activesupport/lib/active_support/subscriber.rb

@@ -88,7 +88,7 @@ def start(name, id, payload)
     end
 
     def finish(name, id, payload)
-      event     = event_stack.pop
+      event = event_stack.pop
       event.finish!
       event.payload.merge!(payload)