difftastic: fix typo in example

The "dark" value actually applies to the `background` setting (which
I’ve omitted here since "dark" is its default value.)

I’ve given `color` the value "always", which will preserve colorization
when using `less` as a pager.

Extracted from `difft --help`¹:

Options:
...
        --color <WHEN>
            When to use color output.

            [env: DFT_COLOR=]
            [default: auto]
            [possible values: always, auto, never]

        --background <BACKGROUND>
            Set the background brightness. Difftastic will prefer brighter colours on dark backgrounds.

            [env: DFT_BACKGROUND=]
            [default: dark]
            [possible values: dark, light]

¹: which seems to be the authoritative source of documentation, as per
https://difftastic.wilfred.me.uk/usage.html#configuration-options

modules/programs/difftastic.nix

@@ -83,7 +83,7 @@ in
         attrsOf (either atom (listOf atom));
       default = { };
       example = {
-        color = "dark";
+        color = "always";
         sort-paths = true;
         tab-width = 8;
       };