Form documentation minor issue

[leobalter]

I've found a minor issue on the Forms documentation in the first example when it says:

 <p>
    <button class="btn-blue">Log in</button>
    <button class="btn">Cancel</button>
</p>

It's missing the btn class in the first button.

 <p>
    <button class="btn btn-blue">Log in</button>
    <button class="btn">Cancel</button>
</p>