Carousel docs example code snippet shouldn't reference Holder.js

Created by: cvrebert

See http://v4-alpha.getbootstrap.com/components/carousel/#example We need to tweak the displayed example code so it doesn't reference Holder.js (data-src="holder.js/900x500/auto/#777:#555/text:First slide") so as to not confuse users. It's fine to use it in the displayed live example though.