Add an example for palette configuration

[averymcnab]

I found this syntax a bit confusing when I first read it, so I thought it would be helpful for the next person to add a short example.

[rhodes-b]

just noting an example already exists in the docs
https://ghostty.org/docs/features/theme#example

you can also look at the default values for examples
ghostty +show-config --default | grep ^palette

[mitchellh]

This is small enough that it make sense to duplicate. Thanks.