Minor grammar change

changed the message returned for 'unique'

Author: bishnudev01

lib/messages.js

@@ -17,5 +17,5 @@ module.exports = {
   required: '"{path}" is required',
   string: '"{path}" must be a string',
   validate: '"{path}" is invalid',
-  unique: '"{path}" is already exists',
+  unique: '"{path}" already exists',
 };