cap_net.3: remove a copypasta

This line appears to have been copied from cap_sysctl.3. While I'm
here, reorder and reword the description of cap_net_limit a bit.

[skip ci]

Sponsored by: Axcient
Reviewed by: oshogbo
Differential Revision: https://reviews.freebsd.org/D42919

(cherry picked from commit 6b96125afdf245ae61dd82b59891ad0d6aab0066)