Update hx-indicator.md

[rafkub]

Fix typo: "add this the" -> "add this to the"

Fix typo - missing "to"

Description

"and this the" -> "and this to the" in the doc.

Corresponding issue: none

Testing

Just the website change.

Checklist

• I have read the contribution guidelines
• I have targeted this PR against the correct branch (master for website changes, dev for
  source changes)
• This is either a bugfix, a documentation update, or a new feature that has been explicitly
  approved via an issue
• [-] [Only doc update, so not needed] I ran the test suite locally (npm run test) and verified that it succeeded