Xr to strtonum(3), because vice versa is already the case

ok jmc@ otto@

src/lib/libc/stdlib/strtol.3

@@ -29,7 +29,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.\"	$OpenBSD: strtol.3,v 1.14 2006/01/13 17:58:09 millert Exp $
+.\"	$OpenBSD: strtol.3,v 1.15 2006/03/29 20:10:52 grunk Exp $
 .\"
 .Dd January 3, 2006
 .Dt STRTOL 3
@@ -239,6 +239,7 @@ The given string was out of range; the value converted has been clamped.
 .Xr atoll 3 ,
 .Xr sscanf 3 ,
 .Xr strtod 3 ,
+.Xr strtonum 3 ,
 .Xr strtoul 3
 .Sh STANDARDS
 The