tracing.7: Shorten doc description for apropos

[ziaee]

Summary

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 (we're not MFCing tracing, right?)

[bcr]

Looks good and it reads better, too.

[0mp]

I wasn't planning to MFC tracing(7). I don't remember if there is anything in the manual that is 15-specific though