Improve formatting for benchmark output (#52106)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/52106

Changelog: [internal]

This slightly improves the formatting of the output produced by benchmarks, so we can just copy&paste the result to share it as valid Markdown.

Reviewed By: christophpurrer

Differential Revision: D76898244

fbshipit-source-id: dc1040ee3787c7f0dcb747c9fba8eb14086a0087