grep: Fix an incorrect description of the -C flag

It seems that the number of lines is no longer an optional parameter to
the -C flag. Document it accordingly both in the manual page and the
usage message.

Reviewed by: yuripv
Approved by: yuripv
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D28509

(cherry picked from commit be6b8b7a3a18acb3b044e144d946392fca05e5d0)