Fix grammatical error in README.md (#253)

Author: Mohak-Trivedi

exercises/04.dom/README.mdx

@@ -22,7 +22,7 @@ There are two ways to use the `ref` prop. A callback and the `useRef` hook.
 
 **ref callback:**
 
-The simplest way is to use the `ref` prop is by passing a callback:
+The simplest way to use the `ref` prop is by passing a callback:
 
 ```tsx
 function MyDiv() {