Fix link to DBIx::Class::ResultSource in README

The README contains this link:

[DBIx::Class::ResultSources](https://metacpan.org/pod/DBIx::Class::ResultSources)

But as far as I can tell, there is no such thing as DBIx::Class::ResultSources. I'm guessing this is what was intended:

[DBIx::Class::ResultSource](https://metacpan.org/pod/DBIx::Class::ResultSource)s