Remove return true on key down function (#2628)

Correct me if I am wrong, but I believe that this `return true` is a no-op. I removed it and everything kept working as usual. And, when I was following the tutorial, that particular line created an interrogation mark in my brain. So, my suggestion is to remove that line to avoid creating entropy for no reason.

docs/walkthroughs/adding-event-handlers.md

@@ -83,7 +83,6 @@ class App extends React.Component {
 
     // Change the value by inserting 'and' at the cursor's position.
     editor.insertText('and')
-    return true
   }
 
   render() {