Use more official link to index-format documentation

This could also be written automatically by labeling it with the
:manpage: role, but it doesn't seem to be an actual manpage, so I
have not done that.

git/index/base.py

@@ -862,7 +862,7 @@ def add(
             handled manually at all.
 
             All current built-in extensions are listed here:
-            http://opensource.apple.com/source/Git/Git-26/src/git-htmldocs/technical/index-format.txt
+            https://git-scm.com/docs/index-format
 
         :return:
             List of :class:`~git.index.typ.BaseIndexEntry`\s representing the entries