Fix behaviour for Parser.handle/2

[hendi]

Running dialyzer results in

The inferred type for the 1st argument is not a
supertype of the expected type for the handle/2 callback
in the Slax.Parser behaviour.

Success type:
%Slax.Event.Characters{_ => _}

Behaviour callback type:
tuple()

This fixes this, by changing the behaviour to the appropriate types.