minor fix in str method
minor fix in str method

pgmpy/pgmpy,jhonatanoliveira/pgmpy,ankurankan/pgmpy,palashahuja/pgmpy,jhonatanoliveira/pgmpy,yashu-seth/pgmpy,abinashpanda/pgmpy,abinashpanda/pgmpy,kris-singh/pgmpy,vivek425ster/pgmpy,sandeepkrjha/pgmpy,pratyakshs/pgmpy,ankurankan/pgmpy,liquidmetal/pgmpy,pgmpy/pgmpy,yashu-seth/pgmpy,anaviltripathi/pgmpy,sandeepkrjha/pgmpy,vivek425ster/pgmpy,khalibartan/pgmpy,kris-singh/pgmpy,khalibartan/pgmpy,anaviltripathi/pgmpy,liquidmetal/pgmpy,pratyakshs/pgmpy,palashahuja/pgmpy