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

Suggested by G. Brandon Robinson.

man/man1/cat.1

@@ -51,7 +51,7 @@ copies to standard output exactly one line from the named
 .IR file ,
 default standard input.
 It is useful in interactive
-.IM rc (1)
+.MR rc (1)
 scripts.
 .PP
 The
@@ -76,11 +76,11 @@ characters and the characters that precede them.
 It is useful to use as
 .B $PAGER
 with the Unix version of
-.IM man (1)
+.MR man (1)
 when run inside a
 .I win
 (see
-.IM acme (1) )
+.MR acme (1) )
 window.
 .SH SOURCE
 .B \*9/src/cmd/cat.c
@@ -89,7 +89,7 @@ window.
 .br
 .B \*9/bin/nobs
 .SH SEE ALSO
-.IM cp (1)
+.MR cp (1)
 .SH DIAGNOSTICS
 .I Read
 exits with status