ktrace.2: correct kern.ktrace.genio_size sysctl name

The man page had kern.ktrace.geniosize but the sysctl node contains an
underscore.

PR: 274274
Reported by: Ivan Rozhuk
Sponsored by: The FreeBSD Foundation

(cherry picked from commit a572dfa1bfe00cec93b27d8848ca49562cab5e3c)