doc: improve readable.unshift example
Checklist
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documentation is changed or added -
the commit message follows commit guidelines
Affected core subsystem(s)
Doc
Description of change
The example would fail to work on a non utf-8 stream because var buf = Buffer.from(remaining, 'utf8');
would introduce unicode replacement characters if anything past the header was not utf-8 encoded. I believe the modified example fixes that issue.