Specify time zone in example code to fix test instability across locales

[timmc]

Addresses https://github.com/Kotlin/kotlinx.serialization/issues/2663

[sandwwraith]

The only thing I would change is to add the comment like

// Here we explicitly set time zone to UTC so output for this sample remains locale-independent. 
// Depending on your needs, you may have to remove this line.

[timmc]

I've pushed up something very similar -- only difference is "adjust or remove".