cbfstool: Fix typo in error message

Change-Id: Iaee7e2c74fe9f63d4d4878278bd445af393942f4
Signed-off-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-on: http://review.coreboot.org/11920
Tested-by: build bot (Jenkins)
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>

util/cbfstool/cbfstool.c

@@ -913,7 +913,7 @@ static int cbfs_update_fit(void)
 static int cbfs_copy(void)
 {
 	if (!param.copyoffset_assigned) {
-		ERROR("You need to specify -D/--copy_offset.\n");
+		ERROR("You need to specify -D/--copy-offset.\n");
 		return 1;
 	}