docs: simplify Transition code example with hooks

Author: silvenon

src/Transition.js

@@ -72,25 +72,23 @@ export const EXITING = 'exiting'
  * begins the "Enter" stage. During this stage, the component will shift from
  * its current transition state, to `'entering'` for the duration of the
  * transition and then to the `'entered'` stage once it's complete. Let's take
- * the following example:
+ * the following example (we'll use the
+ * [useState](https://reactjs.org/docs/hooks-reference.html#usestate) hook):
  *
  * ```jsx
- * state = { in: false };
- *
- * toggleEnterState = () => {
- * this.setState({ in: true });
- * }
- *
- * render() {
+ * function App() {
+ * const [inProp, setInProp] = useState(false);
  * return (
- * <>
- * <Transition in={this.state.in} timeout={500}>
+ * <div>
+ * <Transition in={inProp} timeout={500}>
  * {state => (
  * // ...
  * )}
  * </Transition>
- * <button onClick={this.toggleEnterState}>Click to Enter</button>
- * </>
+ * <button onClick={() => setInProp(true)}>
+ * Click to Enter
+ * </button>
+ * </div>
  * );
  * }
  * ```