Minor updates to README.MD

- changed $ npm install grunt-style-injector to $ npm install grunt-browser-sync (as the plugin name has changed)
- removed the {% highlight bash linenos %} ... {% endhighlight %} wrapper for the plugin install code block (as GitHub Markdown doesn't support this.
- changed $ grunt:prod to $ grunt prod (only way I could get the 'prod' task to run, assume this is a recent syntax change)