[Docs] Updating Custom Seed Sources example for clarity

[jra11]

Instead of

getSeed(seed) {
    switch(seed) {
      case 'seed1':
        return (knex) => { /* ... */ }
    }
  }

It should be:

getSeed(seed) {
    switch(seed) {
      case 'seed1':
        return {
            seed(knex) {
                 /* ... */
            }
        }
    }
  }

Otherwise we get the error "Invalid seed file: seed must have a seed function".

[cuamatzin-govos]

Thank you @jra11

This issue was driving me crazy