[doc] fixed the horizontal form example

The original *horizontal form* example wasn't correctly displayed because it uses `.col-lg-` columns. Using `.col-sm-` columns fixes this problem:

![form-horizontal](https://f.cloud.github.com/assets/73419/1087216/a03d7b90-1611-11e3-8b8c-0e84b4779386.png)

In addition, the `id` attributes of the form controls have been fixed (`inputEmail3` instead of `inputEmail1`, etc.).