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

Suggested by G. Brandon Robinson.

man/man1/deroff.1

@@ -16,13 +16,13 @@ reads each file in sequence
 and removes all
 .I nroff
 and
-.IM troff (1)
+.MR troff (1)
 requests and non-text arguments, backslash constructions,
 and constructs of preprocessors such as
-.IM eqn (1) ,
-.IM pic (1) ,
+.MR eqn (1) ,
+.MR pic (1) ,
 and
-.IM tbl (1) .
+.MR tbl (1) .
 Remaining text is written on the standard output.
 .I Deroff
 follows files included by
@@ -67,7 +67,7 @@ requests.
 Remove titles, attachments, etc., as well as ordinary 
 .IR troff
 constructs, from
-.IM ms (7)
+.MR ms (7)
 or
 .I mm
 documents.
@@ -84,7 +84,7 @@ does for
 and
 .I latex
 (see
-.IM tex (1) )
+.MR tex (1) )
 files what
 .B deroff -wi
 does for
@@ -96,8 +96,8 @@ files.
 .B \*9/src/cmd/delatex.lx
 .SH "SEE ALSO"
 .IR troff (1), 
-.IM tex (1) ,
-.IM spell (1)
+.MR tex (1) ,
+.MR spell (1)
 .SH BUGS
 These filters are not complete interpreters of
 .I troff