Fix symbol (syntax) used for cardinality of 0

When using '0' as described in the README so far,
the following error is reported:

  unexpected "0"
  expecting relationship

(using erd installed from cabal today).

I tried '?', by consistency, which worked,
and checked the source code, which reads:

  let ops = "?1*+"

leaving no doubt.
Merge pull request #3 from medea-project/medea

Fix symbol (syntax) used for cardinality of 0