rc.conf: Fix typo in comment

The correct path is /etc/defaults/rc.conf (defaults in plural).

Reviewed by:	netchild
Fixes:		cc4eb1ea10 ("Add support for a /etc/defaults/vendor.conf override file")
MFC after:	3 days
Differential Revision:	https://reviews.freebsd.org/D56456

libexec/rc/rc.conf

@@ -791,7 +791,7 @@ if [ -z "${source_rc_confs_defined}" ]; then
 	}
 fi
 
-# Allow vendors to override FreeBSD defaults in /etc/default/rc.conf
+# Allow vendors to override FreeBSD defaults in /etc/defaults/rc.conf
 # without the need to carefully manage /etc/rc.conf.
 if [ -r /etc/defaults/vendor.conf ]; then
 	. /etc/defaults/vendor.conf