fix(combobox): associate label with button

[andrewvasilchuk]

In Base Combobox example <label> is associated with non-existed <input> element. The value of for attribute is the id of the <input> element, which is not rendered in DOM. Instead a <button> element is rendered.

The proposed change is to associate label with a button, instead of associating it with a non-existing input element.

[greensteph]

Hello, thanks so much for your contribution. I've created an internal ticket to shepard this through our workflow.