make the shell command clearer

Author: clmath

README.md

@@ -27,7 +27,7 @@ This plugin requires Grunt `~0.4.1`
 If you haven't used [Grunt](http://gruntjs.com/) before, be sure to check out the [Getting Started](http://gruntjs.com/getting-started) guide, as it explains how to create a [Gruntfile](http://gruntjs.com/sample-gruntfile) as well as install and use Grunt plugins. Once you're familiar with that process, you may install this plugin with this command:
 
 ```shell
-npm install grunt-amd-build --save-dev
+$ npm install grunt-amd-build --save-dev
 ```
 
 Once the plugin has been installed, it may be enabled inside your Gruntfile with this line of JavaScript: