Reword repack documentation to no longer state it's a script

This updates the documentation regarding the changes introduced
by a1bbc6c (2013-09-15, repack: rewrite the shell script in C).

Signed-off-by: Stefan Beller <stefanbeller@googlemail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-repack.txt

@@ -14,7 +14,7 @@ SYNOPSIS
 DESCRIPTION
 -----------
 
-This script is used to combine all objects that do not currently
+This command is used to combine all objects that do not currently
 reside in a "pack", into a pack.  It can also be used to re-organize
 existing packs into a single, more efficient pack.