Fix name error in documentation

[Shaeli]

I was trying to rotate a private key following the documentation, it failed with:

/Users/Shaeli/git/github/octocerts/vendor/gems/ruby/3.1.0/gems/acme-client-2.0.13/lib/acme/client.rb:101:in 
`account_key_change': unknown keyword: :private_key (ArgumentError)
        from script/rotate_acme_client_key.rb:12:in `<main>'

because private_key is referenced instead of new_private_key

See here for the method definition

[unixcharles]

Thank you