Doc: Add Optional in Decorator factories

[tomschr]

Situation

The documentation shows an example in the "Decorator factories" section and it looks like this:

# Implementation
def atomic(__func: Callable[..., Any] = None, *, savepoint: bool = True):
    # pruned

However, if you save the file and run mypy, it complains about the previous line:

error: Incompatible default for argument "__func" (default has type "None", argument has type "Callable[..., Any]")  [assignment]
note: PEP 484 prohibits implicit Optional. Accordingly, mypy has changed its default to no_implicit_optional=True

Proposed change

This change adds Optional around Callable[..., Any]] which avoids the above error message.

[hauntsaninja]

Nice, thanks for the fix!