Docs: Incorrect variable name in Schema Builder introduction

[dcordz]

The Schema Builder introduction uses the variable config when it should use databases as per https://github.com/sdispater/orator/issues/77#issuecomment-223693399

Current docs - https://orator-orm.com/docs/0.9/schema_builder.html#introduction and https://orator-orm.com/docs/0.9/basic_usage.html#basicusage

I'm not sure where the docs are kept but I can update the README - https://github.com/sdispater/orator#configuration

I didn't see any contribution guidelines (might have missed them) but I'll open a PR on this issue.