Fix missing space typo in single source error
auto merge of #116 : Indiv0/cargo/typofix, r=alexcrichton

Currently, the "single source" error appears like this, with missing whitespace:

    lib/game-of-life $ cargo build --verbose
    An unknown error occurred

    Caused by:
      At the moment, Cargo only supports asingle source for a particular package name (piston).

This fixes the typo.