DOC: Links to Scipy window functions are out of date

[jpeacock29]

Location of the documentation

• https://github.com/pandas-dev/pandas/blob/master/doc/source/user_guide/computation.rst

• pandas/pandas/core/window/rolling.py

  Line 943 in 00a510b

  <https://docs.scipy.org/doc/scipy/reference/signal.html#window-functions>`__.

Documentation problem

Current link goes to https://docs.scipy.org/doc/scipy/reference/signal.html#window-functions which simply points users to https://docs.scipy.org/doc/scipy/reference/signal.windows.html#module-scipy.signal.windows.

Suggested fix for documentation

Pandas should link directly to the latter documentation.