docs: fix default zlib level

[markwainwright]

This isn't well-covered in the Node docs, but zlib's level actually defaults to zlib.constants.Z_DEFAULT_COMPRESSION (-1), which zlib resolves to 6, not 9:

Z_DEFAULT_COMPRESSION requests a default compromise between speed and compression (currently equivalent to level 6).
(https://www.zlib.net/manual.html#Basic)

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[climba03003]

I would change the value to default as well.

Suggested change

	level: 9, // default is typically 6, max is 9, min is 0
	level: 6, // default is typically 6, max is 9, min is 0

[markwainwright]

Makes sense! Done