[Sync EN] docs(htmlspecialchars): add ENT_SUBSTITUTE example with invalid UTF-8 bytes (#5841)

[github-actions]

Commit: https://github.com/php/doc-en/commit/a9d052e2db27dd26e5f77b13ca03f0e9d352db5a
PR: https://github.com/php/doc-en/pull/5841

Archivos ES a actualizar

• reference/strings/functions/htmlspecialchars.xml