doc: replace methods used in the example code

Methods `buf.writeUIntLE()` and `buf.writeUIntBE()` were used
in the example code for the section of the methods
`writeIntLE()` and `writeIntBE()` instead of the latter.

PR-URL: https://github.com/nodejs/node/pull/16416


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>