wrong providers directory in readme.md for containers

[lluccia]

The instructions in readme are pointing to wrong directory in dockerized keycloak (considering official images quay.io/keycloak/keycloak).

Since version 17, quarkus became the default distribution, so the correct directory is /opt/keycloak/providers instead of /opt/jboss/keycloak/standalone/deployments

https://www.keycloak.org/docs/latest/release_notes/#keycloak-17-0-0

➜  ~ docker run --rm -it --entrypoint bash quay.io/keycloak/keycloak:16.0.0 -c 'ls -ld /opt/keycloak/providers /opt/jboss/keycloak/standalone/deployments'
ls: cannot access '/opt/keycloak/providers': No such file or directory
drwxrwxr-x 2 jboss root 4096 Dec 18  2021 /opt/jboss/keycloak/standalone/deployments

➜  ~ docker run --rm -it --entrypoint bash quay.io/keycloak/keycloak:17.0.1 -c 'ls -ld /opt/keycloak/providers /opt/jboss/keycloak/standalone/deployments'
ls: cannot access '/opt/jboss/keycloak/standalone/deployments': No such file or directory
drwxrwxr-x 2 keycloak root 4096 Mar 23  2022 /opt/keycloak/providers