fix a typo in Queue's docstring

[lmcl90]

No description provided.

[lmcl90]

One UT case fails, but the fixed typo has no relation with this case.

――――――――――――― test_Channel.test_basic_consume_registers_ack_status ―――――――――――――
self = <t.unit.transport.test_pyamqp.test_Channel instance at 0x0000000006a453e0>
    def test_basic_consume_registers_ack_status(self):
        self.channel.wait_returns = 'my-consumer-tag'
        self.channel.basic_consume('foo', no_ack=True)
>       assert 'my-consumer-tag' in self.channel.no_ack_consumers
E       AssertionError: assert 'my-consumer-tag' in set(['y'])
E        +  where set(['y']) = <t.unit.transport.test_pyamqp.Channel object at 0x000000000a7dfda8>.no_ack_consumers
E        +    where <t.unit.transport.test_pyamqp.Channel object at 0x000000000a7dfda8> = <t.unit.transport.test_pyamqp.test_Channel instance at 0x0000000006a453e0>.channel
t/unit/transport/test_pyamqp.py:73: AssertionError
 t/unit/transport/test_pyamqp.py::test_Channel.test_basic_consume_registers_ack_status ⨯60% ██████    
=========================== short test summary info ============================
SKIP [32] /home/travis/build/celery/kombu/.tox/pypy/site-packages/_pytest/nose.py:27: module not installed: pymongo
SKIP [1] /home/travis/build/celery/kombu/.tox/pypy/site-packages/_pytest/nose.py:27: TODO: urllib cannot parse ipv6 urls
SKIP [79] /home/travis/build/celery/kombu/.tox/pypy/site-packages/_pytest/nose.py:27: does not work on PyPy
SKIP [8] /home/travis/build/celery/kombu/.tox/pypy/site-packages/_pytest/nose.py:27: module not installed: etcd
SKIP [13] /home/travis/build/celery/kombu/.tox/pypy/site-packages/_pytest/nose.py:27: module not installed: librabbitmq
Results (41.51s):
     555 passed
       1 failed
         - t/unit/transport/test_pyamqp.py:70 test_Channel.test_basic_consume_registers_ack_status
     133 skipped
ERROR: InvocationError for command '/home/travis/build/celery/kombu/.tox/pypy/bin/pytest -rxs -xv --cov=kombu --cov-report=xml --no-cov-on-fail -v' (exited with code 1)
___________________________________ summary ____________________________________
ERROR:   pypy: commands failed
The command "tox -v -- -v" exited with 1.