Change the macOS-specific suffix from 'os-x' to 'mac'.

This change results in the macOS package being called
'Protege-X.Y.Z-mac.zip' rather than 'Protege-X.Y.Z-os-x.zip', as
suggested by Jim Balhoff since "OS X" is no longer the name used by
Apple to refer to its operating system.

protege-desktop/src/main/assembly/protege-os-x.xml

@@ -2,7 +2,7 @@
           xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
           xsi:schemaLocation="http://maven.apache.org/plugins/maven-assembly-plugin/assembly/1.1.2 http://maven.apache.org/xsd/assembly-1.1.2.xsd">
 
-    <id>os-x</id>
+    <id>mac</id>
     <formats>
         <format>dir</format>
         <format>zip</format>