update file finder url - fixes #699

Author: RonnyPfannschmidt

README.rst

@@ -324,7 +324,7 @@ File finders hook makes most of MANIFEST.in unnecessary
 -------------------------------------------------------
 
 ``setuptools_scm`` implements a `file_finders
-<https://setuptools.readthedocs.io/en/latest/setuptools.html#adding-support-for-revision-control-systems>`_
+<https://setuptools.pypa.io/en/latest/userguide/extension.html#adding-support-for-revision-control-systems>`_
 entry point which returns all files tracked by your SCM. This eliminates
 the need for a manually constructed ``MANIFEST.in`` in most cases where this
 would be required when not using ``setuptools_scm``, namely: