docs: use a valid stylistic set example

[SharakPL]

Summary of the Pull Request

Use example valid for Cascadia font

References

PR Checklist

• Closes Ligatures variants need more explanation #583
• CLA signed. If not, go over here and sign the CLA
• Requires FONTLOG.txt to be updated
• Requires /images/cascadia-code.png and/or /images/cascadia-code-characters.png to be updated
• I've discussed this with core contributors already. If not checked, I'm ready to accept this work might be rejected in favor of a different grand plan. Issue number where discussion took place: Ligatures variants need more explanation #583

Detailed Description of the Pull Request / Additional comments

Description should provide example valid for this font, not use invalid example from FiraCode

Before (if applicable) and After Images of the Character(s)

Validation Steps Performed

[DHowett]

Thanks!