rules: fix grammar in subsystem rule message

Author: evanlucas

lib/rules/subsystem.js

@@ -82,7 +82,7 @@ module.exports = {
       } else {
         context.report({
           id: id
-        , message: 'Release commits does not have subsystems'
+        , message: 'Release commits do not have subsystems'
         , string: ''
         , level: 'skip'
         })