use non-tls ssl? [fix typo]

modules/nixos/zulip.nix

@@ -42,6 +42,6 @@
     };
   };
 
-  # needed fro non-tls connection through port 587
-  services.mailserver.enableSubmissionSsl = true;
+  # needed for non-tls connection through port 587
+  mailserver.enableSubmissionSsl = true;
 }