Reduce confusion about adding additional fields to .this (reactjs#2142)

As a new React learner, this part was a bit confusing as I though that it was referencing `() => this.tick()` part of the code. My addition would help at least people like me.

content/docs/state-and-lifecycle.md

@@ -244,7 +244,7 @@ The `componentDidMount()` method runs after the component output has been render
   }
 ```
 
-Note how we save the timer ID right on `this`.
+Note how we save the timer ID right on `this` (`this.timerID`).
 
 While `this.props` is set up by React itself and `this.state` has a special meaning, you are free to add additional fields to the class manually if you need to store something that doesn’t participate in the data flow (like a timer ID).