fix return type doc in I2C.probe()

[FoamyGuy]

The return type List[int] and declared :rtype: bool in the docstring differ.

This changes the type annotation to match the correct type bool

Confirmed proper type in REPL:

>>> board.I2C().try_lock()
True
>>> board.I2C().probe(0x39)
True

[tannewt]

Thank you!