Make HtmlBuilder and FormBuilder use self-closing tags (also, fix typo)

Using self-closing tags is defined within the HTML5 spec, and is not confined to XHTML only.
Using self-closing tags by default has the benefit of allowing developers to use the Builder classes with XHTML if they choose to, but at the same time has no disadvantage when using HTML5 (and, in fact, still IS HTML5), since it is entirely within the spec.

See [Section 9 of the HTML5 spec](http://www.w3.org/TR/html5/the-xhtml-syntax.html) for more information about self-closing tags in HTML5.

In addition, I fixed a minor typo I found in a comment in the FormBuilder.