Changing reference to <%= target %> in README.md description of backup_to option to something that works

The current description for the backup_to option gives the example of using <%= target %>, however this will throw an error as the target variable won't exist in the context of the current task.

I have changed it to <%= grunt.task.current.target %> which is the [documented way](http://gruntjs.com/inside-tasks#this.target) of accessing this value.