Update example to v13

[Bibo-Joshi]

1. drop use_context=True, it's no longer needed
2. annotate the hello function
3. use f-string b/c v13 dropeed py3.5
4. use update.effective_user instead of updat.message.from_user

'2. is not strictly necessary, but as type hinting is one of the major additions of v13, I think we should showcase it
3.-4. are even more discussable. format still works, but why stick to old syntax? Also update.message.from_user is closer to pure API, while on the other hand PTBs motto is "more than a wrapper" …

[Poolitzer]

3. I love f strings, they are the python spirit imo.
4. I really prefer using effective over anything else, especially because its our library's spirit. Nothing to criticize there.
5. Yes. We absolutely should

[Poolitzer]

LGTM