wam: Use a tuple for the --change-passphrase argument's metavar

Fixes its help entry looking like this:

>  --change-passphrase ADD-ON OLD NEW ADD-ON OLD NEW ADD-ON OLD NEW
>                        Change the passphrase for ADD-ON from OLD to NEW

When it really should look like this:

>  --change-passphrase ADD-ON OLD NEW
>                        Change the passphrase for ADD-ON from OLD to NEW