fix stale docstring in streamz.unique.

[soasme]

I got a TypeError - unexpected keyword argument 'history' when following the document of streamz.unique. It seems the document is stale.

(venv) streamz [doc-fix●●] % python
Python 3.7.4 (default, Jul  9 2019, 18:13:23)
[Clang 10.0.1 (clang-1001.0.46.4)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> from streamz import Stream
>>> source = Stream()
>>> source.unique(history=1).sink(print)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/private/tmp/streamz/streamz/core.py", line 238, in wrapped
    return func(*args, **kwargs)
  File "/private/tmp/streamz/streamz/core.py", line 1228, in __init__
    Stream.__init__(self, upstream, **kwargs)
TypeError: __init__() got an unexpected keyword argument 'history'
>>> source.unique(maxsize=1).sink(print)
<sink: print>

[martindurant]

Thanks!