Fixes monitor representation

[cmilanf]

Monitor.__repr__() was missing a key in the formating that prevented the example of the README.md from this repostory from working.

At Monitor.__init__():
self._fmt = "{}(id={}, name={}, controllable={})"

self._fmt is expecting four keys, but Monitor.__repr__() is only providing three:

def __repr__(self) -> str:
        """Representation of a Monitor."""
        return self._fmt.format(self.__class__.__name__, self.id, self.name)

I just added the missing one:

def __repr__(self) -> str:
        """Representation of a Monitor."""
        return self._fmt.format(self.__class__.__name__, self.id, self.name, self.controllable)