fastfetch: update example for JsonConfig settings

fastfetch: update example for JsonConfig settings

Using the present version of the example, trows an error:

    JsonConfig Error: `display.binaryPrefix` has been renamed to
    `display.size.binaryPrefix`. Sorry for another break change.

It occurs because of change in fastfetch 2.19 of JsonConfig - moving
`display.binaryPrefix` to `display.size.binaryPrefix`

To not confuse the users, this commit changes the example to fit
current standard

See <https://github.com/fastfetch-cli/fastfetch/blob/b3ac696312824cce12df76e398b5362973cde481/CHANGELOG.md?plain=1#L85>

modules/programs/fastfetch.nix

@@ -26,7 +26,9 @@ in {
             };
           };
           display = {
-            binaryPrefix = "si";
+            size = {
+              binaryPrefix = "si";
+            };
             color = "blue";
             separator = "  ";
           };