docs outdated version for ZeroTier

[mikysal78]

Hi ,
this guide refers to the old version of zetotier.
https://openwisp.io/docs/stable/controller/user/zerotier.html
Perhaps it needs updating.
bye bye

[Nabhye]

Hi,
I’d like to work on updating this documentation to reflect the current ZeroTier version.
Please let me know if that’s okay or if you have any specific expectations. Thanks!