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

Suggested by G. Brandon Robinson.

man/man1/ed.1

@@ -96,7 +96,7 @@ beginning of a line.
 supports the
 .I "regular expression"
 notation described in
-.IM regexp (7) .
+.MR regexp (7) .
 Regular expressions are used in addresses to specify
 lines and in one command
 (see
@@ -641,7 +641,7 @@ Dot is unchanged.
 Send the remainder of the line after the
 .L !
 to
-.IM rc (1)
+.MR rc (1)
 to be interpreted as a command.
 Dot is unchanged.
 .TP
@@ -679,9 +679,9 @@ and all characters after the last newline.
 .SH SOURCE
 .B \*9/src/cmd/ed.c
 .SH "SEE ALSO"
-.IM sam (1) ,
-.IM sed (1) ,
-.IM regexp (7)
+.MR sam (1) ,
+.MR sed (1) ,
+.MR regexp (7)
 .SH DIAGNOSTICS
 .BI ? name
 for inaccessible file;