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

Suggested by G. Brandon Robinson.

man/man1/col.1

@@ -14,11 +14,11 @@ and half line feeds (ESC-9 and ESC-8)
 as produced by
 .I nroff
 for .2C in
-.IM ms (7)
+.MR ms (7)
 or
-.IM man (7)
+.MR man (7)
 and for 
-.IM tbl (1) .
+.MR tbl (1) .
 .I Col
 is a pure filter.
 It normally emits only full line feeds;
@@ -47,7 +47,7 @@ paginate the output.
 .SH SOURCE
 .B \*9/src/cmd/col.c
 .SH SEE ALSO
-.IM pr (1)
+.MR pr (1)
 .SH BUGS
 .I Col
 can't back up more than 128 lines or