Fix type documentation about merge_values
Fix type documentation about merge_values

`Haml::AttributeBuilder.merge_attributes!`'s first `to` argument must be
the result of `Parser.parse_class_and_id(attributes)`.

And it will be { String => String } or {}.

If `to` is {}, `merge_value`'s `to` argument can be nil.

haml/haml,haml/haml,haml/haml,haml/haml