nixos/auditd: fix typo

Would otherwise fail with

```
       error: A definition for option `systemd.services.auditd.conflicts."[definition 1-entry 1]"' is not of type `string matching the pattern [a-zA-Z0-9@%:_.\-]+[.](service|socket|device|mount|automount|swap|target|path|timer|scope|slice)'. Definition values:
       - In `/nix/store/x2khl2yx0vz2i357x7mz5xm1kagql8ag-source/nixos/modules/security/auditd.nix': "shutdown.target "
```

Author: mkg20001

nixos/modules/security/auditd.nix

@@ -14,7 +14,7 @@ with lib;
       description = "Linux Audit daemon";
       wantedBy = [ "basic.target" ];
       before = [ "shutdown.target" ];
-      conflicts = [ "shutdown.target "];
+      conflicts = [ "shutdown.target" ];
 
       unitConfig = {
         ConditionVirtualization = "!container";