remove html5-haml from list of included templates

As per my issue request here: https://github.com/middleman/middleman/issues/778

The middleman web page "getting-started" lists the "html5-haml" as being included by default, but the template is not included anywhere that I can see. I had to install the "Middleman-HTML5BP-HAML" template myself from the repo linked-to on that page. If this is not included in the gem, the getting-started page should be changed to reflect that.