treewide: "does not exists" -> "does not exist"

I noticed this minor grammar mistake when running update.nix, and then
while grepping to find the source I noticed we had it a few times in
Nixpkgs.  Just as easy to fix treewide as it was to fix the one
occurrence I noticed.

nixos/modules/services/monitoring/nagios.nix

@@ -41,7 +41,7 @@ let
     validated =  pkgs.runCommand "nagios-checked.cfg" {preferLocalBuild=true;} ''
       cp ${file} nagios.cfg
       # nagios checks the existence of /var/lib/nagios, but
-      # it does not exists in the build sandbox, so we fake it
+      # it does not exist in the build sandbox, so we fake it
       mkdir lib
       lib=$(readlink -f lib)
       sed -i s@=${nagiosState}@=$lib@ nagios.cfg