ifconfig.8: Fix quoting

Obtained from: https://github.com/freebsd/freebsd-src/commit/67d77afb0dde9aa627635a0c4e28a8fa5a64db6c#r69907135
Fixes: 67d77afb0dde ifconfig.8: Do not use ``'' for quoting
MFC after: 2 weeks

(cherry picked from commit 607eed2f84b2c84ef358a98c22467460c423c1a2)