Improve RNTester example for performance.mark/measure (#46759)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/46759

Changelog: [internal]

This improves the example in RNTester for `performance.mark` and `performance.measure` by only listing the marks/measures explicitly emitted from the example. This removes noise if other parts of the app are logging marks/measures as well.

Reviewed By: rshest

Differential Revision: D63695731

fbshipit-source-id: f65cf59363da0e6c9e0b3ffadf05abd2eb5a214c