doc: Use `gmt psconvert` -- new name for ps2raster

doc: Use `gmt psconvert` -- new name for ps2raster

ps2raster isn't found in Ubuntu 18.04 (gmt --version 5.4.3).

Nor in mac/homebrew (gmt --version 6.0.0).

Here's the note that ps2raster is now psconvert:

https://github.com/GenericMappingTools/gmt/blob/master/doc/rst/source/changes.rst#id282

Docs for psconvert command:
https://docs.generic-mapping-tools.org/5.4/psconvert.html