doc: move util.toUSVString() outside of deprecated group

Refs: https://github.com/nodejs/node/pull/39814/files#r693445849

PR-URL: https://github.com/nodejs/node/pull/39840


Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Yongsheng Zhang <zyszys98@gmail.com>
Reviewed-By: Darshan Sen <raisinten@gmail.com>