col-offset-2 to col-sm-offset-2 in bs3-form-horizontal

Hi!

`bs3-form-horizontal` contains this:

```
<div class="col-sm-10 col-offset-2">
  <button type="submit" class="btn btn-primary">Submit</button>
</div>
```

`col-offset-2` doesn't do anything, so updated to `col-sm-offset-2` (which does what I think you intended).

Thanks! Very useful plugin here.
Matt