intel/i40e:fix repeated words in comments

Delete the redundant word 'a'.
Signed-off-by: Jilin Yuan <yuanjilin@cdjrlc.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>

drivers/net/ethernet/intel/i40e/i40e_main.c

@@ -13156,7 +13156,7 @@ static netdev_features_t i40e_features_check(struct sk_buff *skb,
 	}
 
 	/* No need to validate L4LEN as TCP is the only protocol with a
-	 * a flexible value and we support all possible values supported
+	 * flexible value and we support all possible values supported
 	 * by TCP, which is at most 15 dwords
 	 */