Changes variable declarations in code examples (reactjs#2345)

Converts a few `let` declarations for constant values to `const` in code examples

content/docs/hooks-faq.md

@@ -428,8 +428,8 @@ Here, we store the previous value of the `row` prop in a state variable so that
 
 ```js
 function ScrollView({row}) {
-  let [isScrollingDown, setIsScrollingDown] = useState(false);
-  let [prevRow, setPrevRow] = useState(null);
+  const [isScrollingDown, setIsScrollingDown] = useState(false);
+  const [prevRow, setPrevRow] = useState(null);
 
   if (row !== prevRow) {
     // Row changed since last render. Update isScrollingDown.
@@ -718,7 +718,7 @@ As a last resort, if you want something like `this` in a class, you can [use a r
 ```js{2-6,10-11,16}
 function Example(props) {
   // Keep latest props in a ref.
-  let latestProps = useRef(props);
+  const latestProps = useRef(props);
   useEffect(() => {
     latestProps.current = props;
   });