Update install guide for Debian

Hello,

as Debian 11 was released on 2021-08-14 I want to ask if it is possible to update the corresponding wiki entry for Freepbx 16 to the new version.

Also I want to point out a small issue regarding the autostart-script linked at the very end. The link leads to nowhere and should be corrected.

@psandesh

Best regards,

Afox