Fix formatting of __borrow_guard_selftest description

[Shawon-Dey188]

What does this PR do?

Checklist

• make test passes locally
• New builtins have signature comments and docs in docs/BUILTINS.md
• New library functions follow conventions in docs/STDLIB.md
• New examples have a comment header explaining what they demonstrate
• CHANGELOG.md updated (if user-facing change)

[InauguralPhysicist]

Thanks for the eye on formatting! Closing this one, though: the trailing | on that row is deliberate — every row in every table in this file (and the repo's other docs) closes with a pipe, so removing it here would make this one row inconsistent rather than fixing it. GitHub renders both forms identically, so there's no rendering bug to chase.

(Heads-up: your #553 accidentally touches this same line the other way — adding trailing spaces — worth dropping that hunk there too. Review with details is on #553; looking forward to landing it.)