difftastic: fix example: s/sort-path/sort-paths/

Fixed the example, I realized it was wrong since I copied it and got an error:

```
$ difft --sort-path
error: unexpected argument '--sort-path' found

  tip: a similar argument exists: '--sort-paths'
```

The option is documented in `difft --help` and implemented in `difftastic/src/options.rs`.

7d8175dccf/src/options.rs (L375)

It's used correctly in the jujutsu submodule at the end of this file.

modules/programs/difftastic.nix

@@ -75,7 +75,7 @@ in
       default = { };
       example = {
         color = "dark";
-        sort-path = true;
+        sort-paths = true;
         tab-width = 8;
       };
       description = "Configuration options for {command}`difftastic`. See {command}`difft --help`";