Fix Heading (#537)

Author: zackify

content/guides/hmr-react.md

@@ -235,7 +235,7 @@ The important thing to note in the code above is the `module` reference.
 
 So in this case, `module.hot.accept` will fire the `render` method whenever `src/components/App.js` or its dependencies are changed - which means the `render` method will also fire when the `App.css` is changed, since `App.css` is included in `App.js`.
 
-###index.html
+### index.html
 
 This needs to be placed inside of `dist` in your project root. webpack-dev-server will not run without it.