As chapter "24.4. Using DTrace" describes dtrace scripts hotkernel and procsystime are under /usr/share/dtrace path. But now it is a part of sysutils/dtrace-toolkit port and located in /usr/local/share/dtrace-toolkit
A commit references this bug:
Date: Sat May 9 10:11:07 UTC 2020
New revision: 54122
Correct the location of the dtrace-toolkit when
installed from ports/packages.
Submitted by: Alexandr Krivulya
Thank you for sending this problem report. I just committed a fix to the handbook.