Switch unless to if.

Co-authored-by: Mike McQuaid <mike@mikemcquaid.com>

Library/Homebrew/rubocops/shared/desc_helper.rb

@@ -65,7 +65,7 @@ module RuboCop
         problem "Description shouldn't start with the #{type} name." if regex_match_group(desc, /^#{name_regex}\b/i)
 
         if type == :cask && match = regex_match_group(desc, /\b(macOS|Mac( ?OS( ?X)?)?|OS ?X)\b/i)
-          problem "Description shouldn't contain the platform." unless match[1] == "MAC"
+          problem "Description shouldn't contain the platform." if match[1] != "MAC"
         end
 
         # Check if a full stop is used at the end of a desc (apart from in the case of "etc.").