doc: correct my wrong note about buf.fill()
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
documentation is changed or added -
commit message follows commit guidelines
Sorry, I've misinterpreted this operation:
https://github.com/nodejs/node/blob/c1ac57888199ba13df7eda4912cdb53dcfb5a2ee/src/node_buffer.cc#L566
> Buffer.alloc(5).fill(255)
<Buffer ff ff ff ff ff>
> Buffer.alloc(5).fill(256)
<Buffer 00 00 00 00 00>
> Buffer.alloc(5).fill(257)
<Buffer 01 01 01 01 01>
> Buffer.alloc(5).fill(258)
<Buffer 02 02 02 02 02>