Use const for ref in functional component docs (#1129)

Co-authored-by: Sunil Pai <threepointone@fb.com>

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 = useRef(null);
+  const textInput = useRef(null);
   
   function handleClick() {
     textInput.current.focus();