doc: fix typo in TextEncoding section

PR-URL: https://github.com/nodejs/node/pull/18201


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Khaidi Chu <i@2333.moe>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Weijia Wang <starkwang@126.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>