Fix bsdgrep manpage clobbering grep(1) with default build options

The default build should install bsdgrep(1) and grep(1), with the latter
being gnugrep(1). WITH_BSD_GREP flips this situation such that we have
gnugrep(1) and grep(1), with the latter being bsdgrep(1).

Changes to start installing the zgrep script out of usr.bin/grep
inadvertently altered the default build such that grep(1) was being
installed, and it was bsdgrep(1). Correct the typo.

Reported by: bcran
MFC after: 3 days