Fix test target code of module_function (#1316)

The test target code was wrong. `module_function` is only available
inside module, not inside class.

`module_function` is `undef`ed in Class.
```ruby
Class.undefined_instance_methods
=> [:prepend_features, :extend_object, :refine, :module_function, :append_features]
```

Author: tompng

test/rdoc/test_rdoc_parser_prism_ruby.rb

@@ -1023,17 +1023,17 @@ class A
 
   def test_module_function
     util_parser <<~RUBY
-      class A
+      module A
         def m1; end
         def m2; end
         def m3; end
         module_function :m1, :m3
         module_function def m4; end
       end
     RUBY
-    klass = @store.find_class_named 'A'
-    instance_methods = klass.method_list.reject(&:singleton)
-    singleton_methods = klass.method_list.select(&:singleton)
+    mod = @store.find_module_named 'A'
+    instance_methods = mod.method_list.reject(&:singleton)
+    singleton_methods = mod.method_list.select(&:singleton)
     if accept_legacy_bug?
       instance_methods.last.visibility = :private
       singleton_methods << singleton_methods.last.dup