doc: add await to browser.close in usage examples

[cohesively]

Since f398e69 updated browser.close to return a promise, the usage examples should be updated to reflect the new signature.

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[JoelEinbinder]

Looks good!

[aslushnikov]

@cohesively could you please sign CLA? We'll merge after that

[cohesively]

I signed it!

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CLAs look good, thanks!