doc: correct introduced_in metadata for buffer doc

The buffer module was introduced in v0.1.90. Updated `introduced_in`
value in `buffer.md` to reflect this.

PR-URL: https://github.com/nodejs/node/pull/19545


Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>