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 <b3ac696312/CHANGELOG.md (L85)>
