fix: config example sslmode to require, not required

[TakabayaP]

sslmode is set to required in the example of dbconfig.yml in the readme, but sslmode has to be one of "require", "verify-full", "verify-ca", or "disable" (https://github.com/lib/pq/blob/381d253611d666974d43dfa634d29fe16ea9e293/ssl.go#L14).

[rubenv]

Thanks!