docs: add trackBy hint to repeat util

[Pascalmh]

When I added the repeat util to my code I was wondering if I still had to add a TrackByFunction as well - after checking the code I now know that it's not needed.

This change should clarify this for other users.

[nx-cloud]

☁️ Nx Cloud Report

CI is running/has finished running commands for commit 608e38e. As they complete they will appear below. Click to see the status, the terminal output, and the build insights.

📂 See all runs for this CI Pipeline Execution

---

⌛ The following targets are in progress

• nx affected --target=build --parallel=3 --exclude=docs
• nx affected --target=test --parallel=3 --ci --code-coverage
• nx affected --target=lint --parallel=3

✅ Successfully ran 1 target

• nx-cloud record -- pnpm exec nx format:check

---

Sent with 💌 from NxCloud.