docs(api.md): use async/await instead of then

[randombenj]

fixes #5082

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[randombenj]

@googlebot I signed it!

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[mathiasbynens]

This now no longer matches the current Puppeteer coding style.

[mathiasbynens]

Suggested change

	for (let child of frame.childFrames()) {
	for (const child of frame.childFrames()) {

[mathiasbynens]

Thanks!