tty: serial: Kconfig: Fix a typo

'exsisting' has an extra 's'
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Link: https://lore.kernel.org/r/20200216102742.19298-1-christophe.jaillet@wanadoo.frSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/Kconfig

@@ -1111,7 +1111,7 @@ config SERIAL_SC16IS7XX_SPI
 	help
 	  Enable SC16IS7xx driver on SPI bus,
 	  If required say y, and say n to spi if not required,
-	  This is additional support to exsisting driver.
+	  This is additional support to existing driver.
 	  You must select at least one bus for the driver to be built.
 
 config SERIAL_TIMBERDALE