doc: fix Apple Silicon Homebrew prefix change documentation

Commit 4c8d654 incorrectly stated that Homebrew has changed its
prefix for Apple M1 machines, but the prefix change applies to all
Apple Silicon based machines.  Fix by writing Apple Silicon instead
of Apple M1.

Reported-by: Dagfinn Ilmari Mannsåker <ilmari@ilmari.org>
Discussion: https://postgr.es/m/87mt3ys8ng.fsf@wibble.ilmari.org

Author: danielgustafsson

doc/src/sgml/docguide.sgml

@@ -213,7 +213,7 @@ brew install docbook docbook-xsl libxslt fop
 <programlisting>
 export XML_CATALOG_FILES=/usr/local/etc/xml/catalog
 </programlisting>
-    On Apple M1 based machines, use this:
+    On Apple Silicon based machines, use this:
 <programlisting>
 export XML_CATALOG_FILES=/opt/homebrew/etc/xml/catalog
 </programlisting>