netback: fix typo in comment

"variables a used" should be "variables are used".
Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/xen-netback/netback.c

@@ -395,7 +395,7 @@ static void netbk_gop_frag_copy(struct xenvif *vif, struct sk_buff *skb,
 	struct gnttab_copy *copy_gop;
 	struct netbk_rx_meta *meta;
 	/*
-	 * These variables a used iff get_page_ext returns true,
+	 * These variables are used iff get_page_ext returns true,
 	 * in which case they are guaranteed to be initialized.
 	 */
 	unsigned int uninitialized_var(group), uninitialized_var(idx);