DOC: Correct Series.nonzero docstring

The function returns indices, not a boolean array. The description now
matches `numpy.nonzero`.
It just depends if you want to refer to a toplevel function or an array method. 
And in this case, they both exists, but the docs of `np.nonzero` is more elaborate (`np.ndarray.nonzero` also just refers to `np.nonzero`). So for here, changing to `numpy.nonzero` is good.