Add additional example for 'spo theme set'

Currently, for the command [spo theme set](https://pnp.github.io/cli-microsoft365/cmd/spo/theme/theme-set#examples) we only show examples with an @-token file path. Let's add an additional example where we specify the entire JSON-payload as option value.