sysctl.9: Fix grammar in *TUN descriptions

Reported by: bcr
Reviewed by: bcr
MFC after: 3 days
Event: EuroBSDCon 2025
Fixes: 5eb1caa8c845 Updated SYSCTL manual pages
Differential Revision: https://reviews.freebsd.org/D52768

(cherry picked from commit f34e1c76adc5efd9b603cf6b2d5827f7db8c2ed8)