adjust --env help text for `doublezero confg` from mainnet to mainnet-beta

[dalalama]

the help suggests mainnet vs mainnet beta for doublezero config --env

Set a config setting

Usage: doublezero config set [OPTIONS] <--env |--url |--ws |--keypair |--program-id <PROGRAM_ID>>

Options:
--env DZ env shorthand to set the config to (testnet, devnet, or mainnet)