update docstring of _get_ancestors_of method
update docstring of _get_ancestors_of method

ankurankan/pgmpy,khalibartan/pgmpy,liquidmetal/pgmpy,abinashpanda/pgmpy,pgmpy/pgmpy,sandeepkrjha/pgmpy,kris-singh/pgmpy,anaviltripathi/pgmpy,jhonatanoliveira/pgmpy,kris-singh/pgmpy,jhonatanoliveira/pgmpy,anaviltripathi/pgmpy,liquidmetal/pgmpy,khalibartan/pgmpy,pgmpy/pgmpy,sandeepkrjha/pgmpy,abinashpanda/pgmpy,ankurankan/pgmpy