Documentation: restore a space in unpack-objects usage

The commit 87b7b841 removed a space in the unpack-objects usage, which
makes the synopsis a bit confusing. This patch simply restores it.
Signed-off-by: NVivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/git-unpack-objects.txt

@@ -9,7 +9,7 @@ git-unpack-objects - Unpack objects from a packed archive.
 
 SYNOPSIS
 --------
-'git-unpack-objects' [-q] <pack-file
+'git-unpack-objects' [-q] < pack-file
 
 
 DESCRIPTION