Add synonym SF6 to sulphur hexafluoride

[f-innatech]

Description of the issue

sulphur hexafluoride should have a synonym SF6.

Ideas of solution

see above.

Workflow checklist

• [ ] I discussed the issue with someone else than me before working on a solution
• [ ] I already read the latest version of the workflow for this repository
• [ ] The goal of this ontology is clear to me

I am aware that

• [ ] every entry in the ontology should have a definition
• [ ] classes should arise from concepts rather than from words