git-update-ref.txt: add missing word

The description for the "verify" command is lacking a single word "is",
which this commit corrects.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-update-ref.txt

@@ -107,7 +107,7 @@ delete::
 
 verify::
 	Verify <ref> against <oldvalue> but do not change it.  If
-	<oldvalue> zero or missing, the ref must not exist.
+	<oldvalue> is zero or missing, the ref must not exist.
 
 option::
 	Modify behavior of the next command naming a <ref>.