git-apply(1): Clarify that one can select where to apply the patch

The patch can be applied to the work tree, the index or both, but the
short description made it look like it's always applied to both.

Signed-off-by: Björn Steinbrink <B.Steinbrink@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-apply.txt

@@ -3,7 +3,7 @@ git-apply(1)
 
 NAME
 ----
-git-apply - Apply a patch on a git index file and a working tree
+git-apply - Apply a patch on a git index file and/or a working tree
 
 
 SYNOPSIS