git-prune-packed.txt: fix reference to GIT_OBJECT_DIRECTORY

git-prune-packed operates on GIT_OBJECT_DIRECTORY, not
GIT_OBJECT_DIR.
Signed-off-by: NSteffen Prohaska <prohaska@zib.de>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/git-prune-packed.txt

@@ -14,7 +14,7 @@ SYNOPSIS
 
 DESCRIPTION
 -----------
-This program searches the `$GIT_OBJECT_DIR` for all objects that currently
+This program searches the `$GIT_OBJECT_DIRECTORY` for all objects that currently
 exist in a pack file as well as the independent object directories.
 
 All such extra objects are removed.