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

Suggested by G. Brandon Robinson.

man/man1/tr2post.1

@@ -15,7 +15,7 @@ converts
 .I files
 (or standard input),
 which should be the device-independent output of
-.IM troff (1) ,
+.MR troff (1) ,
 into the PostScript printer language.
 .PP
 The options are:
@@ -47,11 +47,11 @@ logical pages per physical page
 Using this option emits PostScript with invalid document structuring
 comments.
 It will print fine but will not view correctly in
-.IM gv (1)
+.MR gv (1)
 or
 .I psv
 (see
-.IM page (1) ).
+.MR page (1) ).
 .TP
 .BI -o " pagelist
 Print only the pages in the
@@ -110,5 +110,5 @@ psv /tmp/a.ps
 .SH SOURCE
 .B \*9/src/cmd/postscript/tr2post
 .SH SEE ALSO
-.IM troff (1) ,
-.IM psfonts (1)
+.MR troff (1) ,
+.MR psfonts (1)