doc: remove inspector experimental warning

PR-URL: https://github.com/nodejs/node/pull/12408
Ref: https://github.com/nodejs/node/pull/12352


Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>