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: Author: bcr Date: Sat May 9 10:11:07 UTC 2020 New revision: 54122 URL: https://svnweb.freebsd.org/changeset/doc/54122 Log: Correct the location of the dtrace-toolkit when installed from ports/packages. PR: 246302 Submitted by: Alexandr Krivulya Changes: head/en_US.ISO8859-1/books/handbook/dtrace/chapter.xml
Thank you for sending this problem report. I just committed a fix to the handbook. PR closed.