update URL to the search plugins site

http://gulpjs.com/plugins/ probably be an "official"
search plugins website as it filters out black-listed ones.

Ex.: http://gulpjs.com/plugins/ shows 602 plugins vs. > 800 plugins on http://gratimax.net/search-gulp-plugins/

Not sure which site is the "proper" one, but it would be good to have an "official" pointer in gulp FAQ