mpe.4: properly refer to ioctl(2)

[Caspar Schutijser]

Index: mpe.4
===================================================================
RCS file: /cvs/src/share/man/man4/mpe.4,v
retrieving revision 1.11
diff -u -p -r1.11 mpe.4
--- mpe.4       18 Mar 2021 14:22:04 -0000      1.11
+++ mpe.4       14 May 2021 10:01:58 -0000
@@ -41,7 +41,7 @@ The interface itself can be configured w
 see its manual page for more information.
 .Sh IOCTLS
 The following
-.Nm ioctl 2
+.Xr ioctl 2
 calls are specific to
 .Nm ,
 .Xr mpip 4