docs: add javascript syntax highlighting to 'Usage in Jest'

Author: yokinist

README.md

@@ -120,7 +120,7 @@ renderToNodeStream(app)
 ## Usage in Jest
 While testing in using jest, if there is a need to emulate SSR, the following string is required to have the test behave the way they are expected to.
 
-```
+```javascript
 import { HelmetProvider } from 'react-helmet-async';
 
 HelmetProvider.canUseDOM = false;