Input: Fix spelling mistake in Kconfig "useable" -> "usable"

There is a spelling mistake in the Kconfig text. Fix it.
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Link: https://lore.kernel.org/r/20210705100230.7583-1-colin.king@canonical.comSigned-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

drivers/input/keyboard/Kconfig

@@ -213,7 +213,7 @@ config KEYBOARD_LKKBD
 	select SERIO
 	help
 	  Say Y here if you want to use a LK201 or LK401 style serial
-	  keyboard. This keyboard is also useable on PCs if you attach
+	  keyboard. This keyboard is also usable on PCs if you attach
 	  it with the inputattach program. The connector pinout is
 	  described within lkkbd.c.