Add a line break to make separate paragraphs

At present, the position of "*Deprecated*" statement in the `grunt.util._` section seems to be different from `grunt.util.async` and `grunt.util.hooker`'s.

I think "*Deprecated*" should be in another paragraph separated from the next sentence "Lo-dash and ...".

Here is the screenshot of http://gruntjs.com/api/grunt.util#external-libraries:

![screenshot-2013-12-16-2 47 21](https://f.cloud.github.com/assets/1131567/1750764/e2b3296c-65b1-11e3-8b8b-37dd73f680af.jpg)