Move pretty_name into MacOS::Version

Closes Homebrew/homebrew#20507.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>

Library/Homebrew/macos.rb

@@ -20,7 +20,7 @@ module MacOS extend self
   end
 
   def pretty_name
-    cat.to_s.split('_').map(&:capitalize).join(' ')
+    MacOS.version.pretty_name
   end
 
   def locate tool

Library/Homebrew/os/mac/version.rb

@@ -13,5 +13,9 @@ module MacOS
           end
       super(Version.new(v))
     end
+
+    def pretty_name
+      @version.split('_').map(&:capitalize).join(' ')
+    end
   end
 end