Fix command for serving docs locally (#8)

Author: ProfessorLogout

content/contributing/docs.md

@@ -27,7 +27,7 @@ feature.
 ## Building Docs
 
 To build the docs locally, run `tox r -e docs`. To serve the docs locally, run
-`python -m http.server docs/_build/dirhtml`. Then go to <https://localhost:8000>
+`python -m http.server -d docs/_build/dirhtml`. Then go to <https://localhost:8000>
 in your browser to view the docs.
 
 Documentation is hosted by Read the Docs. It is deployed on every change to the