Docs: fix env var for enabling query

[gkeeth]

The docs referred to QUERY_PORT as the right env var for enabling query in the server properties, but as noted later in the docs that's for configuring the port for query (maps to the query.port server property), not for enabling/disabling query.

The correct env var to reference here is ENABLE_QUERY, I think. Ref https://github.com/itzg/docker-minecraft-server/blob/master/files/property-definitions.json#L12

[itzg]

Thanks, that's indeed what it should have been.