agent/raft: fix typo in help strings

[jasonodonnell]

Small error in the vault operator raft join CLI helper text.

$ vault operator raft join -h
...
Command Options:

  -leader-ca-cert=<string>
      CA cert to communicate with Raft leader.

  -leader-client-cert=<string>
      Client cert to to authenticate to Raft leader.

  -leader-client-key=<string>
      Client key to to authenticate to Raft leader.
...

[alexanderbez]

LGTM 🎉