Fix a spec failure

```
1)
Warning.[]= :experimental emits and suppresses warnings for :experimental FAILED
Expected "" =~ /is experimental/
to be truthy but was nil
```

spec/ruby/core/warning/element_set_spec.rb

@@ -14,7 +14,7 @@ ruby_version_is '2.7' do
         end
 
         ruby_version_is "3.0" do
-          @src = '[0, 1] => [a, b]'
+          @src = 'warn "This is experimental warning.", category: :experimental'
         end
       end