doc: correct my wrong note about buf.fill()

[vsemozhetbyt]

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• documentation is changed or added
• commit message follows commit guidelines

Sorry, I've misinterpreted this operation:

node/src/node_buffer.cc

Line 566 in c1ac578

int value = val & 255;

> 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>

[nodejs-github-bot]

@vsemozhetbyt build started: https://ci.nodejs.org/blue/organizations/jenkins/node-test-pull-request-lite-pipeline/detail/node-test-pull-request-lite-pipeline/2302/pipeline

[vsemozhetbyt]

Node.js Collaborators, please, add 👍 here if you approve fast-tracking.

[vsemozhetbyt]

Landed in 17c6b1d
Thank you for the reviews.