rpcbind.8: Fix missing option entry in commit df3f247e4696

Commit df3f247e4696 added descriptions for the new command line
option "-I". However, it failed to update the .Op line.

This is a content change.

Reviewed by: Alexander Ziaee <concussious.bugzilla@runbox.com> (manpages)
Differential Revision: https://reviews.freebsd.org/D45118