docs: modify gm world tutorial to use docker cmd for auth token

[jcstein]

based on #231 (comment)

[jcstein]

note to add this when issue resolved and this works:

First, set your auth token:

export CELESTIA_NODE_AUTH_TOKEN=$(docker exec $(docker ps -q)  celestia bridge --node.store /bridge  auth admin)
echo $CELESTIA_NODE_AUTH_TOKEN

Next, check your balance:

docker exec $(docker ps -q) celestia rpc state Balance --node.store /bridge