Update grdimage example to use recommended syntax

[maxrjones]

Description of the problem

The current documentation for grdimage includes an example that uses three grids as input:

To create an false color plot from the three grid files red.nc, green.nc, and blue.nc, with
linear scaling at 10 inch/x-unit, tickmarks every 5 units:

gmt grdimage red.nc green.nc blue.nc -Jx10i -B5 -pdf rgbimage

The use of grdimage with three grid files as input is no longer recommended/documented (following discussion in #4044).

It would be good to update this example to include a call to grdmix first to produce the image that is provided to grdimage.