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

Suggested by G. Brandon Robinson.

man/man1/mc.1

@@ -18,10 +18,10 @@ splits the input into as many columns as will fit in
 .I N
 print positions.
 If run in a
-.IM 9term (1) ,
-.IM xterm (1) ,
+.MR 9term (1) ,
+.MR xterm (1) ,
 or
-.IM acme (1)
+.MR acme (1)
 window, the default
 .I N
 is the number of blanks that will fit across the window;
@@ -36,14 +36,14 @@ is printed separately.
 .SH SOURCE
 .B \*9/src/cmd/draw/mc.c
 .SH "SEE ALSO"
-.IM 9term (1) ,
-.IM acme (1) ,
-.IM acme (4) ,
-.IM xterm (1) ,
-.IM pr (1) ,
+.MR 9term (1) ,
+.MR acme (1) ,
+.MR acme (4) ,
+.MR xterm (1) ,
+.MR pr (1) ,
 .I lc
 in
-.IM ls (1)
+.MR ls (1)
 .SH BUGS
 On systems with high-DPI screens,
 .I 9term