docs: fix grammar in quicksilver variables

[G-Rath]

Summary

Quicksilver Variables - Fix incorrect word usage.

Dependencies and Timing

Release:

• [x] When ready
• [ ] ~After date: $DATE~

---

Post Launch

Do not remove - To be completed by the docs team upon merge:

• [ ] Redirect /old-path/ => /new-path/ (if applicable)
• [ ] Include/exclude pages ^ respectively within docs search service provider (if applicable)
• [ ] For Heroes - add a props post to the discussion board.
• [ ] Remove from the project board