Fix typo in example in docs that throws an error.

[mmartin313]

The example in the doc has a typo. If the code is copied the user will receive the error:

Traceback (most recent call last):
  File "run.py", line 35, in <module>
    print api.healthcheck.get()
  File "/Users/xxx/.virtualenvs/aaq/lib/python2.7/site-packages/slumber/__init__.py", line 155, in get
    resp = self._request("GET", params=kwargs)
  File "/Users/xxx/.virtualenvs/aaq/lib/python2.7/site-packages/slumber/__init__.py", line 90, in _request
    headers = {"accept": serializer.get_content_type()}
  File "/Users/xxx/.virtualenvs/aaq/lib/python2.7/site-packages/slumber/serialize.py", line 109, in get_content_type
    return s.get_content_type()
  File "/Users/xxx/.virtualenvs/aaq/lib/python2.7/site-packages/slumber/serialize.py", line 26, in get_content_type
    raise NotImplementedError()
NotImplementedError