upb: fix a Ruby CI test

PiperOrigin-RevId: 624002294

ruby/tests/basic.rb

diff --git a/ruby/tests/basic.rb b/ruby/tests/basic.rb
index a45e600..a55f921 100755
--- a/ruby/tests/basic.rb
+++ b/ruby/tests/basic.rb
@@ -716,11 +716,11 @@
       message.freeze
 
       assert_raise FrozenError do
-        message.map_string_msg["message"].foo = "bar"
+        message.map_string_msg["message"].foo = nested_message_2
       end
 
       assert_raise FrozenError do
-        message.repeated_msg[0].foo = "bar"
+        message.repeated_msg[0].foo = nested_message_2
       end
     end