Use current main official link to git-clone URLS doc

I am still not using the :manpage: role for this because it points
to a specific section, which that role does not support, and it is
important that it go to that specific section. (See d8ab99c for
details.)

Author: EliahKagan

git/repo/base.py

@@ -1488,8 +1488,7 @@ def clone_from(
         """Create a clone from the given URL.
 
         :param url:
-            Valid git url, see:
-            http://www.kernel.org/pub/software/scm/git/docs/git-clone.html#URLS
+            Valid git url, see: https://git-scm.com/docs/git-clone#URLS
 
         :param to_path:
             Path to which the repository should be cloned to.