[BUG / mc] Update mc idp ldap accesskey create

[djwfyi]

mc idp ldap accesskey create can take a second parameter not described in the docs.
Update the docs to reflect the third example in the help output.

Create a new access key pair for user with username "james" that expires in 1 day                                   
     $ mc idp ldap accesskey create local/ james --expiry-duration 24h   

Where james is the target user for which the accesskey is created.

Goals

• Update the syntax to reflect the additional second parameter
• Add the example above to the docs as a use case