update aux-1 pins on ramps

[ellensp]

Description

While looking around the ramps 1.4 pins I noticed that the circuit diagram on reprap.org was wrong
see https://reprap.org/mediawiki/images/archive/f/f6/20250316110106%21RAMPS1.4schematic.png
In the top right of the MEGA Conn section it has RX - D1 and TX - D2 this is in error
It actually RX - D0 and TX - D1 (I have also confirmed this on a real physical ramps 1.4)

See https://docs.arduino.cc/retired/hacking/hardware/PinMapping2560/ for conformation

Pin Number	Pin Name	Mapped Pin Name
2	    PE0 ( RXD0/PCINT8 )	Digital pin 0 (RX0)
3	    PE1 ( TXD0 )	Digital pin 1 (TX0)

On the circuit diagram this was again copied in error to the AUX-1 Section

This was in turn copied into marlin for AUX-1

This PR fixes the pins in Marlin, and I have submitted a corrected circuit diagram image https://reprap.org/mediawiki/images/f/f6/RAMPS1.4schematic.png

Requirements

Anything that uses Marlin/src/pins/ramps/pins_RAMPS.h

Benefits

Accurate documentation.

[thinkyhead]

I'll be sure to add this to all the LTS branches also.