mailmap doc: use correct environment variable 'GIT_WORK_TREE'

gitmailmap(5) uses 'GIT_WORK_DIR' to refer to the root of the
repository, but this environment variable does not exist.

Use the correct spelling for that variable, 'GIT_WORK_TREE'.

Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/gitmailmap.txt

diff --git a/Documentation/gitmailmap.txt b/Documentation/gitmailmap.txt
index 7f10897..052209b 100644
--- a/Documentation/gitmailmap.txt
+++ b/Documentation/gitmailmap.txt
@@ -7,7 +7,7 @@
 
 SYNOPSIS
 --------
-$GIT_WORK_DIR/.mailmap
+$GIT_WORK_TREE/.mailmap
 
 
 DESCRIPTION