fitsemivariance.py: typo in spherical function's docstring

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