Docs reference an import of setup_context_menu that doesn't exist

[herronelou]

NodeGraphQt/docs/examples/ex_menu.rst

Line 67 in a8fa9b3

from NodeGraphQt import BaseNode, NodeGraph, setup_context_menu

module 'NodeGraphQt' has no attribute 'setup_context_menu'

[jchanvfx]

closing this issue as it a duplicate of #455