Update help information in cardano-cli output

[Jimbo4350]

We now how Era based commands instead of COMMAND and I have removed "Shelley" in the help output of all the Era based commands.

Usage: cardano-cli (Era based commands | Byron specific commands | 
                     Miscellaneous commands)

Available options:
  --version                Show the cardano-cli version
  -h,--help                Show this help text

Era based commands
  address                  Payment address commands
  stake-address            Stake address commands
  ...

Byron specific commands
  byron                    Byron specific commands

Miscellaneous commands
  version                  Show the cardano-cli version

[newhoggy]

Would be good to paste an example of help output in these sorts of PRs, but the code itself looks good.

[Jimbo4350]

bors merge

[iohk-bors]

Build succeeded:

• buildkite/cardano-node
• ci/hydra-eval
• ci/hydra-build:required