simplify file naming

https://github.com/edmondchuc/ontogram/tree/main/examples:
I have some suggestions to simplify the file extensions. In particular, the established PlantUML extension is `.puml`.

- tern-org.ttl.txt -> tern-org.puml
- tern-org.ttl.txt.png - tern-org.png
- tern-org.ttl.txt.svg -> tern-org.svg