Fixes text the `postal_code` test.
Fixes text the `postal_code` test.

Makes sure the test passes only if the `postal_code` methods returns
exactly four digits.

Thanks @marocchino

teeparham/ffaker,ffaker/ffaker,44uk/ffaker,vveliev/ffaker,ffaker/ffaker,dugjason/ffaker,vveliev/ffaker,44uk/ffaker,EmmanuelOga/ffaker,EmmanuelOga/ffaker,esender/ffaker,teeparham/ffaker,dugjason/ffaker,esender/ffaker