parameter is missing in the signature of function in the stumpy document

[Nima Sarajpoor]

Please see this:

https://stumpy.readthedocs.io/en/latest/api.html#stumpy.gpu_stump

The parameter k is in the docstring but not in the signature of the function.

[Sean M. Law]

For any function that is GPU related, the signature must be updated manually in docs/api.rst

I wonder if there is a way to automate this?

[Sean M. Law]

@NimaSarajpoor I updated the docstrings and also preemptively fixed the other two GPU functions as well in #880