nixos/tmpfiles: fix docs

Closes #381822

Apparently, I swapped `path` and `tmpfiles-type` in
2be50b1efe. Sorry about that 🫠

Also giving
`systemd.tmpfiles.settings.<config-name>.<path>.<tmpfiles-type>.type` a
better default in the manual than `‹name›`, i.e. `‹tmpfiles-type›` so
that it corresponds to the placeholders in the attribute path.

nixos/modules/system/boot/systemd/tmpfiles.nix

@@ -29,10 +29,11 @@ let
       };
     };
     default = {};
-    type = attrsWith' "config-name" (attrsWith' "tmpfiles-type" (attrsWith' "path" (types.submodule ({ name, config, ... }: {
+    type = attrsWith' "config-name" (attrsWith' "path" (attrsWith' "tmpfiles-type" (types.submodule ({ name, config, ... }: {
       options.type = mkOption {
         type = types.str;
         default = name;
+        defaultText = "‹tmpfiles-type›";
         example = "d";
         description = ''
           The type of operation to perform on the file.