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

Suggested by G. Brandon Robinson.

man/man1/eqn.1

@@ -15,7 +15,7 @@ eqn  \- typeset mathematics
 .SH DESCRIPTION
 .I Eqn
 is a
-.IM troff (1)
+.MR troff (1)
 preprocessor
 for typesetting mathematics
 on a typesetter.
@@ -34,7 +34,7 @@ named in the
 option (default
 .BR -Tutf ;
 see
-.IM troff (1) ).
+.MR troff (1) ).
 When run with other preprocessor filters,
 .I eqn
 usually comes last.
@@ -299,7 +299,7 @@ Mathematical words like
 .LR cos ,
 .L log
 are made Roman automatically.
-.IM Troff (1)
+.MR Troff (1)
 four-character escapes like
 .L \e(lh
 (\(lh) can be used anywhere.
@@ -319,7 +319,7 @@ font descriptions for PostScript
 .B \*9/src/cmd/eqn
 .SH "SEE ALSO"
 .IR troff (1), 
-.IM tbl (1)
+.MR tbl (1)
 .br
 J. F. Ossanna and B. W. Kernighan,
 ``Troff User's Manual''.