Fix listener example in C++ documentation.

[kosak]

The listener example still refers to Ref<> and shared pointers, but in
reality the generated method does not pass a shared pointer. The actual
signature looks more like this:

  virtual void enterKey(ParserRuleContext * /*ctx*/) override { }

This change updates the example to match that signature, and removes a line
discussing Ref<>. It also adds the override keyword in order to make the
user's code a little more robust against typos/changes in the grammar.