Fix the pattern for changing PermitRootLogin

The previous pattern had the (harmless) side-effect of duplicating the
entry in the config file.

No functional change intended.

Reviewed by: imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/510

(cherry picked from commit 73358cd2cfccf9aafde069ffcb8185a0f3420ffd)