Fix a typo in the BC CredentialsProvider test

@rsvoboda Spotted a typo in the test - the reason it works without a typo is because currently the default keystore password is `password` which matches the `server-keystore.jks` password.
I've confirmed that changing either `keystore-password` or `truststore-password` keys in `SecretProvider` fails the tests.
For whatever reasons, if I try to change the `server-keystore.jks` password (just to ensure that no default keystore value is picked up), it messes up the `server-keystore.jks` and Vertx can't read it, with or without `CredentialsProvider`.
Rostislav, if you'd like, please confirm independently that changing the passwords in `SecretProvider` causes a failure