[docs] venv module doc missing command line option

[C.D. MacEachern]

The documentation for venv on both 3.6.4rc1 and 3.7 is missing a command
line option:

When I run my python 3.6.3 with: 

$ python -m venv -h

There is the long-dash option:
--prompt PROMPT Provides an alternative prompt prefix for this
environment.

There is no mention in the above python doc pages about this in the
28.3.1 section, however there is a brief mention of it in the 28.3.2 API
section.

Thanks,
Craig MacEachern



-- 
  C.D. MacEachern
 craigmaceachern at fastmail.com