docs: modbus: Change pymodbus unit= parameter to slave=

The PyModbus project changed the `unit=` parameter to `slave=` in the
v3.3.0 release.

See https://pymodbus.readthedocs.io/en/latest/source/api_changes.html#api-changes-3-3-0



Signed-off-by: Chris Wilson <chris@cgnd.dev>