Updating Sad Face to Happy Face

[hannahchapman79]

I have a slight issue with Rainbow-Poem /Cambodia.txt I think there should be a smiley face at the end rather than a sad face.

[github-actions[bot]]

Thank you for contributing and using this repository while learning GitHub. We have fallen behind on new issues and are resolving them in the order they were submitted. It maybe a number of weeks before we get to this one. Sorry for any inconvenience this causes. Choo choo! 🚂🌈❤️