dtrace.1: Mention providers in SEE ALSO

Reviewed by: bnovkov, christos, markj
Approved by: bnovkov (mentor), christos (mentor), markj (mentor)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D50852

(cherry picked from commit 177471148a6111537e84912fb9a033b6ac5ecb17)