boards: atsamd20_xpro: fix tx pin number for sercom3

Not pretty to change the PAD name from 2 to 1 as the correct
is PAD2 but for the SAMD20 the register value is 1.

Fixes: 6d08958a ("drivers: uart_sam0: move sercom pad info to dts")
Signed-off-by: Sean Nyekjaer <sean.nyekjaer@prevas.dk>