Update documentation for running vault in development mode

[kamalhm]

Hi, I'm a newbie on vault and docker but I'd like to suggest changing this command $ docker run --cap-add=IPC_LOCK -e 'VAULT_DEV_ROOT_TOKEN_ID=myroot' -e 'VAULT_DEV_LISTEN_ADDRESS=0.0.0.0:1234' vault on the documentation to include the expose port arguments, and also set the vault listen_address to port 8200 to reduce the friction for someone to play around with vault in docker.

Here's my proposed command, I'd be willing to make a PR if it's welcome
$ docker run -p 8200:8200 --cap-add=IPC_LOCK -e 'VAULT_DEV_ROOT_TOKEN_ID=myroot' -e 'VAULT_DEV_LISTEN_ADDRESS=0.0.0.0:8200' vault