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

@@ -7,7 +7,7 @@ gitmailmap - Map author/committer names and/or E-Mail addresses
 
 SYNOPSIS
 --------
-$GIT_WORK_DIR/.mailmap
+$GIT_WORK_TREE/.mailmap
 
 
 DESCRIPTION