Updating name of variable to which returned value of setState is assigned (#3701)

in CodePen it is 'prevState' and in the tutorial, it is 'state', which creates confusion.

content/docs/handling-events.md

@@ -71,8 +71,8 @@ class Toggle extends React.Component {
   }
 
   handleClick() {
-    this.setState(state => ({
-      isToggleOn: !state.isToggleOn
+    this.setState(prevState => ({
+      isToggleOn: !prevState.isToggleOn
     }));
   }