doc: adding example to Buffer.isBuffer method

PR-URL: https://github.com/nodejs/node/pull/36233


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Juan José Arboleda <soyjuanarbol@gmail.com>