Add missing word in previous CSS selector documentation

[gritvald]

Description

I've added the missing word in the previous <CSS selector> documentation.

Checklist

• [x] I have read the contribution guidelines
• [x] I have targeted this PR against the correct branch (master for website changes, dev for source changes)
• [x] This is either a bugfix, a documentation update, or a new feature that has been explicitly approved via an issue
• [x] I ran the test suite locally (npm run test) and verified that it succeeded

[Telroshan]

Thanks!