Fixing pass parameter channel with message EVENT_CHANNEL_MEMBERS_UPDATE

[hypnocapybara]

Documentation says, that we get this parameters (3):

EVENT_CHANNEL_MEMBERS_UPDATE = 'members_update' """When users join/leave a channel

:param channel: channel instance
:type  channel: :class:`ChatChannel`
:param joined: list of members who joined
:type  joined: list
:param left: list of members who left
:type  left: list
"""

But in real, only joined and left passed. A little fix, to pass channel too

[rossengeorgiev]

Nice spot thanks