update readme

Author: benweizhu

README.md

@@ -4,7 +4,7 @@ A React Component based on [heatmap.js][b7fa289f].
 
   [b7fa289f]: https://www.patrick-wied.at/static/heatmapjs/ "heatmap.js"
 
-```
+```javascript
 import React from 'react';
 import ReactDom from 'react-dom';
 import ReactHeatmap from 'react-heatmap';