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.

3 files changed, 3 insertions, 3 deletions

diff --git a/nixos/modules/services/backup/znapzend.nix b/nixos/modules/services/backup/znapzend.nix
index debb2a39705..1fccc7cd607 100644
--- a/nixos/modules/services/backup/znapzend.nix
+++ b/nixos/modules/services/backup/znapzend.nix
@@ -324,7 +324,7 @@ in
       autoCreation = mkOption {
         type = bool;
         default = false;
-        description = "Automatically create the destination dataset if it does not exists.";
+        description = "Automatically create the destination dataset if it does not exist.";
       };
 
       zetup = mkOption {
diff --git a/nixos/modules/services/monitoring/nagios.nix b/nixos/modules/services/monitoring/nagios.nix
index 0afaefe04e1..280a9a001b5 100644
--- a/nixos/modules/services/monitoring/nagios.nix
+++ b/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
diff --git a/nixos/modules/services/networking/firefox/sync-server.nix b/nixos/modules/services/networking/firefox/sync-server.nix
index 24f76864953..1ad573abfca 100644
--- a/nixos/modules/services/networking/firefox/sync-server.nix
+++ b/nixos/modules/services/networking/firefox/sync-server.nix
@@ -119,7 +119,7 @@ in
           password, and the <option>syncserver.secret</option> setting is used by the server to
           generate cryptographically-signed authentication tokens.
 
-          If this file does not exists, then it is created with a generated
+          If this file does not exist, then it is created with a generated
           <option>syncserver.secret</option> settings.
        '';
       };