C#: Replace FuncRef with Callable in outdated example

[raulsntos]

• FuncRef was removed in 4.0.
• The GDScript examples were updated in rework two funcref code examples #6542.

[paulloz]

LGTM.
Note: it might be better to use ChildType.PropertyNames.FuncProperty instead of "FuncProperty", but it's probably not that big of a deal.

[raulsntos]

Note: it might be better to use ChildType.PropertyNames.FuncProperty instead of "FuncProperty", but it's probably not that big of a deal.

Agreed, but this whole page uses strings everywhere already. To be honest, I think the C# examples in this page should be rewritten to avoid strings and using Get/Set/Call/Connect when possible. But I will leave that for a future PR.

[mhilbrunner]

Thanks! Merged.