tracing.7: Shorten doc description for apropos

Shorten document description to fit on one line for the beautification
of apropos results, removing only the search keyword "facilities", which
I think is unlikely to cause discoverability regression.

MFC after: No (please mfc if mfc'ing tracing.7)
Reviewed by: 0mp, bcr
Differential Revision: https://reviews.freebsd.org/D51284