Fix documentation for connecting to multiple devservice databases

[albers]

I had some difficulties connecting to the devservices databases of a service with multiple datasources because the documentation in the Dev Services for Databases guide is wrong.

How to verify

Inspect the devservice database containers of a multi-datasource application with docker inspect.

Here are the env vars of the postgres container for the named datasource "db1":

$ docker container inspect unruffled_franklin --format '{{json .Config.Env}}' | jq -r '.[]' | grep POSTGRES
POSTGRES_USER=quarkus
POSTGRES_PASSWORD=quarkus
POSTGRES_DB=db1

For reference, these are the corresponding env vars from a devservices container for the default database in a single-datasource setup:

$ docker container inspect clever_murdock --format '{{json .Config.Env}}' | jq -r '.[]' | grep POSTGRES
POSTGRES_USER=quarkus
POSTGRES_PASSWORD=quarkus
POSTGRES_DB=quarkus

This PR fixes the issue.

[github-actions]

🎊 PR Preview 4198ee4 has been successfully built and deployed to https://quarkus-pr-main-45801-preview.surge.sh/version/main/guides/

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