Documentation: Ardupilot "SERIALx_BAUD" instead of "SERIALx_BAUDRATE"

[NikosSprocket]

Parameter name correction for setting up the Baud rate of the Rx serial port.

At the 3rd paragraph on page
https://www.expresslrs.org/quick-start/ardupilot-setup/?h=ardupilot
the Ardupilot parameter for the CRSF option should be

SERIALx_BAUD

instead of

SERIALx_BAUDRATE

[Nikhil Parikh]

fixed: 0cf9dbb