doc: bump ICU version to avoid confusion

ICU has been on 6X for a while but example file name in Intl docs
still refers to it as 5X. It might be a small thing but I think
it's good to update this to avoid confusion.

PR-URL: https://github.com/nodejs/node/pull/22313


Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: George Adams <george.adams@uk.ibm.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>