specified language to fix the language detection problem in modal docs

Syntax highlighting is not working in advanced code snippet of modal
http://foundation.zurb.com/apps/docs/#!/modal

Because of custom angular attributes, highlight.js could not detect html language type in few code snippets
I fixed only modal docs where I found a syntax detection problem.

It is better to define all angular code snippets in
```
<hljs language='html'>
<hljs>
```