Documentation: receive.denyCurrentBranch defaults to 'refuse'

acd2a45b (Refuse updating the current branch in a non-bare repository
via push, 2009-02-11) changed the default to refuse such a push, but
it forgot to update the docs.
Signed-off-by: NThomas Rast <trast@student.ethz.ch>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -1442,7 +1442,7 @@ receive.denyCurrentBranch::
 	out of sync with the index and working tree. If set to "warn",
 	print a warning of such a push to stderr, but allow the push to
 	proceed. If set to false or "ignore", allow such pushes with no
-	message. Defaults to "warn".
+	message. Defaults to "refuse".
 
 receive.denyNonFastForwards::
 	If set to true, git-receive-pack will deny a ref update which is