Fix command for serving docs locally

[ProfessorLogout]

Nothing major, just a small bug in the command shown to serve the docs of a project after building.

Example, using the click project:

With the current command:

$ python -m http.server docs/_build/dirhtml

usage: server.py [-h] [--cgi] [-b ADDRESS] [-d DIRECTORY] [-p VERSION] [port]
server.py: error: argument port: invalid int value: 'docs/_build/dirhtml'

With the fixed command:

$ python -m http.server -d docs/_build/dirhtml

Serving HTTP on :: port 8000 (http://[::]:8000/) ...