doc: add "chrony" to preflight checklist for Ubuntu 18.04

doc: add "chrony" to preflight checklist for Ubuntu 18.04

This commit updates the "Install NTP" section of the preflight
checklist, instructing readers who are using Ubuntu 18.04 to
use the package name "chrony" where elsewhere the package name
"ntp" would be used. "chrony" has replaced "ntp" in Ubnutu 18.04.

This commit fixes Bug number 8 in the list here:
https://pad.ceph.com/p/Report_Documentation_Bugs

Signed-off-by: Zac Dover <zac.dover@gmail.com>