Rephrase paragraph about externalComponents

When a customer decides to externalize specific databases apimanager object must reflect their decision according to the https://github.com/3scale/3scale-operator/blob/3scale-2.13-stable/doc/apimanager-reference.md#externalcomponentsspec that is if customer decides to externalize only backend redis database they need to set in their apimanager object spec.externalComponents.backend.redis = true. When they want to externalize backend redis and system database they must set both spec.externalComponents.backend.redis = true and spec.externalComponents.system.database = true. etc

Reported by: ficap_devel

https://access.redhat.com/documentation/en-us/red_hat_3scale_api_management/2.13/html/installing_3scale/install-threescale-on-openshift-guide#apimanager_custom_resources_to_deploy_3scale