Add example to Any documentation

Noticed there wasn't an awful lot of info out there on using Any types, so added an example to the rustdocs.
Oh, huh. Sorry for the contradiction.

I looked around and some places use `##`, others use `#`, I filed https://github.com/rust-lang/rust/issues/15499 and based on the loose counts there, it seems `#` is more common, so may we can go with that here.

(Thanks for writing docs. :heart:)