fitsemivariance.py: typo in spherical function's docstring

[SimonMolinsky]

Function spherical has a small typo in docstring. Variable c0 is described as a sill and variable c is described as a nugget but they should be described inversely:

def spherical(h, c0, c, a):
    """
    Input
      h: distance
      c0: nugget
      c: sill
      a: range
    """