git.txt: list index versions in plain English

At the first look, a user may think the default version is "23". Even
with UNIX background, there's no reference anywhere close that may
indicate this is glob or regex.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git.txt

@@ -739,7 +739,8 @@ Git so take care if using Cogito etc.
 'GIT_INDEX_VERSION'::
 	This environment variable allows the specification of an index
 	version for new repositories.  It won't affect existing index
-	files.  By default index file version [23] is used.
+	files.  By default index file version 2 or 3 is used. See
+	linkgit:git-update-index[1] for more information.
 
 'GIT_OBJECT_DIRECTORY'::
 	If the object storage directory is specified via this