Fix description of DictProperty's defaultvalue
Fix description of DictProperty's defaultvalue

The DictProperty's defaultvalue actually defaults to {} (see
test_dictcheck() in test_properties.py for example). Updated the
docstring to contain the correct information.

matham/kivy,matham/kivy,inclement/kivy,akshayaurora/kivy,inclement/kivy,matham/kivy,rnixx/kivy,kivy/kivy,rnixx/kivy,kivy/kivy,akshayaurora/kivy,rnixx/kivy,akshayaurora/kivy,kivy/kivy,matham/kivy,inclement/kivy