mt76: mt7615: fix up typo in Kconfig for MT7663U

Fix up typo in Kconfig with indicating MT7663U is an 802.11ac device

Fixes: eb99cc95 ("mt76: mt7615: introduce mt7663u support")
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>

drivers/net/wireless/mediatek/mt76/mt7615/Kconfig

@@ -35,6 +35,6 @@ config MT7663U
 	depends on MAC80211
 	depends on USB
 	help
-	  This adds support for MT7663U 802.11ax 2x2:2 wireless devices.
+	  This adds support for MT7663U 802.11ac 2x2:2 wireless devices.
 
 	  To compile this driver as a module, choose M here.