Minor fix to setup.cfg

[iameskild]

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Changes introduced in this PR:

• Minor fix to setup.cfg
  Replace long_description_type with long_description_content_type

Based on error from publishing RC:

Checking dist/qhub-0.4.0rc0-py3-none-any.whl: FAILED
  `long_description` has syntax errors in markup and would not be rendered on PyPI.
    line 3: Warning: Definition list ends without a blank line; unexpected unindent.
  warning: `long_description_content_type` missing. defaulting to `text/x-rst`.
Checking dist/qhub-0.4.0rc0.tar.gz: FAILED
  `long_description` has syntax errors in markup and would not be rendered on PyPI.
    line 3: Warning: Definition list ends without a blank line; unexpected unindent.
  warning: `long_description_content_type` missing. defaulting to `text/x-rst`.

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[tonyfast]

lgtm. sorry for the misspelling