nixos/manual: Fix build
nixos/manual: Fix build

In fd9eb16b249aad1d5e231b8329035abfab5fc0eb, the option
"services.nextcloud.nginx.enable" has been removed since the module now
exclusively supports nginx only.

Unfortunately, with the option gone from the manual, the link in the
Nextcloud-specific documentation referencing the NixOS option also
became a dead link and thus the manual will no longer build.

I also removed a second reference to this option in the Nextcloud-
specific documentation, which while it doesn't lead to a build error in
the manual is nevertheless a good idea to remove as well to ensure we
don't present outdated information to readers of the manual.

Signed-off-by: aszlig <ee1aa092358634f9c53f01b5a783726c9e21b35a@nix.build>
Cc: b2e4cf94038f795b8f77f43b1995727b4bc74807@Ma27

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs