doc: Fix XML_CATALOG_FILES env var for Apple M1 machines
Homebrew changed the prefix for Apple M1 based machines, so our
advice for XML_CATALOG_FILES needs to mention both. More info
on the Homebrew change can be found at:
https://github.com/Homebrew/brew/issues/9177
Author: Julien Rouhaud <julien.rouhaud@free.fr>
Discussion: https://postgr.es/m/
20230327082441.h7pa2vqiobbyo7rd@jrouhaud