doc: change v-notation for version in buffer.md

Change v6 to 6.0.0. We abandoned v-notation for versions to avoid
confusion between v8 (version 8.0.0) and V8 (the JavaScript engine).

PR-URL: https://github.com/nodejs/node/pull/19567


Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>