restore a missing space in start command

Error introduced in ab0b96bb80ac, reported at
<https://github.com/krbullock/vlad-unicorn/issues/7>.

--HG--
extra : amend_source : f328804ab23453df6a5f8584065c17c617656df9
prepare to release 2.3.1
tagging v2.3.1 for release