Fix for Issue 186 - Replace ref to `table-fields` in README with `entity-fields`

`table-fields` doesn't exist anymore, and so trying out Korma using the README documentation results in errors.

The function was replaced by `entity-fields` in commit d75d1e1f70a14940809e1a6722e5879d2e7bbb96, this PR just changes the README to match the code.