Clarify and clean up language around a package vendor name

Clarify and clean up language around a package vendor name and its relationship with PSR-0. (hint: there is none)

Happy to tweak this more if we want changes. I modified the wording from [Package Names](http://getcomposer.org/doc/01-basic-usage.md#package-names) section of the Composer documentation.

https://groups.google.com/d/msg/php-fig/JdR-g8ZxKa8/ifKiM-8P0zAJ