Fix: correct dns upstreams `FTLCONF_dns_upstreams` variable name

[shamoon]

Description

I couldn't figure out why the container wasnt starting because the log output says to use FTLCONF_dns_upstream (missing the s) which was pretty confusing 😅, I think its just a typo

Motivation and Context

Error suggests setting an incorrect environment variable

How Has This Been Tested?

This compose fails and gives the output:

  piholev6:
      container_name: piholev6
      image: pihole/pihole:development-v6
      ports:
        - "53:53/tcp"
        - "53:53/udp"
        - "80:80/tcp"
      environment:
        TZ: 'America/Los_Angeles'
        FTLCONF_webserver_api_password: thisisapassword
        FTLCONF_dns_upstream: 8.8.8.8;1.1.1.1
        FTLCONF_dns_listeningMode: all

Verified the variable name at https://ftl.pi-hole.net/development-v6/docs/#get-/config and of course it makes the docker container run

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.