Add another good example for RelativeDateConstant

The current good example seems to discourage the usage of a constant
completely. Add another good example to showcase how to continue using a
constant and have things working dynamically.

Author: ngan

lib/rubocop/cop/rails/relative_date_constant.rb

@@ -14,6 +14,15 @@ module Rails
       #
       #   # good
       #   class SomeClass
+      #     EXPIRES = 1.week
+      #
+      #     def self.expired_at
+      #       EXPIRES.since
+      #     end
+      #   end
+      #
+      #   # good
+      #   class SomeClass
       #     def self.expired_at
       #       1.week.since
       #     end

manual/cops_rails.md

@@ -1900,6 +1900,15 @@ class SomeClass
   EXPIRED_AT = 1.week.since
 end
 
+# good
+class SomeClass
+  EXPIRES = 1.week
+
+  def self.expired_at
+    EXPIRES.since
+  end
+end
+
 # good
 class SomeClass
   def self.expired_at