Fix logger call for gcode multi commands events

[Diaoul]

Missing _ causes the whole thing to fail.

Traceback (most recent call last):
  File "/home/pi/OctoPrint/src/octoprint/events.py", line 110, in _work
    listener(event, payload)
  File "/home/pi/OctoPrint/src/octoprint/events.py", line 267, in eventCallback
    self.executeCommand(processedCommand, commandType)
  File "/home/pi/OctoPrint/src/octoprint/events.py", line 275, in executeCommand
    self._executeGcodeCommand(command)
  File "/home/pi/OctoPrint/src/octoprint/events.py", line 296, in _executeGcodeCommand
    self.logger.debug("Executing GCode commands: %r" % command)
AttributeError: 'CommandTrigger' object has no attribute 'logger'

[foosel]

Apparently I forgot to backport f5eef06 unto master