Alert dismissable fix example

Created by: NinoD

Just a small fix in docs. http://getbootstrap.com/components/#alerts-dismissable

<div class="alert alert-dismissable">

should have .alert-contextual

<div class="alert alert-warning alert-dismissable">

In your own words: "No default class Alerts don't have default classes, only base and modifier classes. A default gray alert doesn't make too much sense, so you're required to specify a type via contextual class. Choose from success, info, warning, or danger."