docs: multiplayer env variable correction

[davidaparicio]

As discussed in the issue #199, I found an error to add the environment variable as it was written ;-)

In fact, the environmentVariables map wasn't correct set

[davidaparicio]

The error was cannot patch "retool-multiplayer-ws" with kind Deployment: Deployment.apps "retool-multiplayer-ws" is invalid: spec.template.spec.containers[0].env[13].name: Invalid value: "0": a valid environment variable name must consist of alphabetic characters, digits, '_', '-', or '.', and must not start with a digit (e.g. 'my.env-name', or 'MY_ENV.NAME', or 'MyEnvName1', regex used for validation is '[-._a-zA-Z][-._a-zA-Z0-9]*')

The 13th value was the mine in my values.yaml, defined as it was in the documentation :)

environmentVariables:
- name: MAIN_DOMAIN
  value: retool.foo.com