Fix ref creation in CustomTextInput component (#2072)

Change React.createRef() to useRef(null)

Author: myronyeung

content/docs/refs-and-the-dom.md

@@ -168,7 +168,7 @@ You can, however, **use the `ref` attribute inside a function component** as lon
 ```javascript{2,3,6,13}
 function CustomTextInput(props) {
   // textInput must be declared here so the ref can refer to it
-  let textInput = React.createRef();
+  let textInput = useRef(null);
 
   function handleClick() {
     textInput.current.focus();