Fix typo in `replace` example

[36degrees]

Summary

Quote the string aaabbbccc so that it is treated as a string rather than a variable name.

Add a space before the closing delimiter to match the format used elsewhere on the page.

Checklist

I've completed the checklist below to ensure I didn't forget anything. This makes reviewing this PR as easy as possible for the maintainers. And it gets this change released as soon as possible.

• [x] Proposed change helps towards purpose of this project.
• [x] Documentation is added / updated to describe proposed change.
• [ ] Tests are added / updated to cover proposed change. N/A
• [ ] Changelog has an entry for proposed change (if user-facing fix or feature). N/A