Remove broken link to NumPy's HOWTO_DOCUMENT.rst.txt (pandas-dev#51010)

Author: eriknw

doc/source/development/contributing_docstring.rst

@@ -67,8 +67,6 @@ case of pandas, the NumPy docstring convention is followed. These conventions ar
 explained in this document:
 
 * `numpydoc docstring guide <https://numpydoc.readthedocs.io/en/latest/format.html>`_
-  (which is based in the original `Guide to NumPy/SciPy documentation
-  <https://github.com/numpy/numpy/blob/main/doc/HOWTO_DOCUMENT.rst.txt>`_)
 
 numpydoc is a Sphinx extension to support the NumPy docstring convention.