Update probability.py

[zhuyuanxiang]

hi, maybe I found a error in probability.py.
At Line 1907, "from matplotlib.pyplot import plt" maybe is the right code.

[stevenbird]

NB #2317

[BLaZeKiLL]

this still causes the import error
replacing it with
import matplotlib.pyplot as plt
causes a different error
UnboundLocalError: local variable 'v' referenced before assignment

I have fixed it (at least it is working for me now) #2319

[stevenbird]

Thanks @zhuyuanxiang - we'll accept a different PR for this