doc: clarify protocol default in http.request()

The previously listed default of 'http' is incorrect, and
causes an error to be thrown. This commit changes it to the
correct value of 'http:'

Fixes: https://github.com/nodejs/node/issues/4712
PR-URL: https://github.com/nodejs/node/pull/4714


Reviewed-By: James M Snell <jasnell@gmail.com>