BayesianModel.py: Modified docstring of BayesianModel class
BayesianModel.py: Modified docstring of BayesianModel class

Bayesian model accepts any hashable object while
docstring mentions it accepts only Strings.So there
was a need to modify the docstring.

Fixes #https://github.com/pgmpy/pgmpy/issues/823

pgmpy/pgmpy,khalibartan/pgmpy,jhonatanoliveira/pgmpy,jhonatanoliveira/pgmpy,khalibartan/pgmpy,ankurankan/pgmpy,pgmpy/pgmpy,ankurankan/pgmpy