Fix a spec failure

```
1)
The -W command line option with :no-experimental suppresses experimental warnings FAILED
Expected "" =~ /is experimental/
to be truthy but was nil
```

spec/ruby/command_line/dash_upper_w_spec.rb

@@ -37,7 +37,7 @@ 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