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

Suggested by G. Brandon Robinson.

man/man1/tbl.1

@@ -11,7 +11,7 @@ tbl \- format tables for nroff or troff
 is a preprocessor for formatting tables for
 .I nroff
 or
-.IM troff (1) .
+.MR troff (1) .
 The input
 .I files
 are copied to the standard output,
@@ -74,7 +74,7 @@ recognize
 and
 .I y
 as
-.IM eqn (1)
+.MR eqn (1)
 delimiters
 .PD
 .RE
@@ -275,8 +275,8 @@ Bernardsville	2018	3.30
 .B \*9/src/cmd/tbl
 .SH SEE ALSO
 .IR troff (1), 
-.IM eqn (1) ,
-.IM doctype (1)
+.MR eqn (1) ,
+.MR doctype (1)
 .br
 M. E. Lesk and L. L. Cherry,
 ``TBL\(ema Program to Format Tables'',