remove logical typo in documentation of sample update hook

Signed-off-by: Heiko Voigt <hvoigt@hvoigt.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/githooks.txt

@@ -245,7 +245,7 @@ Both standard output and standard error output are forwarded to
 for the user.
 
 The default 'update' hook, when enabled--and with
-`hooks.allowunannotated` config option turned on--prevents
+`hooks.allowunannotated` config option unset or set to false--prevents
 unannotated tags to be pushed.
 
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