Add `URI#open` to warning message

lib/open-uri.rb

@@ -15,7 +15,7 @@ module Kernel
        (name.respond_to?(:to_str) &&
         %r{\A[A-Za-z][A-Za-z0-9+\-\.]*://} =~ name &&
         (uri = URI.parse(name)).respond_to?(:open))
-      warn('calling URI.open via Kernel#open is deprecated, call URI.open directly', uplevel: 1)
+      warn('calling URI.open via Kernel#open is deprecated, call URI.open directly or use URI#open', uplevel: 1)
       URI.open(name, *rest, **kw, &block)
     else
       open_uri_original_open(name, *rest, **kw, &block)