Updated documentation with syntax highlighting. (#9448)

Updated documentation with syntax highlighting.

docs/src/pages/guides/guide-react-native/index.md

@@ -63,7 +63,7 @@ Create a file called `rn-addons.js` that you can use to include on device addons
 
 `storybook/rn-addons.js`
 
-```
+```js
 import '@storybook/addon-ondevice-knobs/register';
 import '@storybook/addon-ondevice-notes/register';
 ...