Make it clear Root#initializer doesn't yield block

It will make ruby point to the correct source of the warning.

ref #109

lib/dry/initializer/mixin/root.rb

@@ -7,8 +7,8 @@ module Mixin
       module Root
         private
 
-        def initialize(...)
-          __dry_initializer_initialize__(...)
+        def initialize(*, **)
+          __dry_initializer_initialize__(*, **)
         end
       end
     end