nixos/hedgedoc: add fchown to SystemCallFilter to fix filesystem backend

Author: Lassulus

nixos/modules/services/web-apps/hedgedoc.nix

@@ -327,6 +327,7 @@ in
           "@system-service"
           "~@privileged @obsolete"
           "@pkey"
+          "fchown" # needed for filesystem image backend
         ];
         UMask = "0007";
       };