tmac: rename IM (italic manual) to MR (manual reference)

Suggested by G. Brandon Robinson.

man/man3/quaternion.3

@@ -121,7 +121,7 @@ The following routines operate on rotations or orientations represented as unit
 .TP
 .B mtoq
 Convert a rotation matrix (see
-.IM matrix (3) )
+.MR matrix (3) )
 to a unit quaternion.
 .TP
 .B qtom
@@ -148,12 +148,12 @@ This is just a rotation about the same axis by half the angle.
 .SH SOURCE
 .B \*9/src/libgeometry/quaternion.c
 .SH SEE ALSO
-.IM matrix (3) ,
-.IM qball (3)
+.MR matrix (3) ,
+.MR qball (3)
 .SH BUGS
 To avoid name conflicts with NetBSD,
 .I qdiv
 is a preprocessor macro defined as 
 .IR p9qdiv ;
 see
-.IM intro (3) .
+.MR intro (3) .