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

Suggested by G. Brandon Robinson.

man/man1/diff.1

@@ -19,7 +19,7 @@ two directories are compared by the method of
 .I diff
 for text
 files and
-.IM cmp (1)
+.MR cmp (1)
 otherwise.
 If more than two file names are given, then each argument is compared
 to the last argument as above.
@@ -140,9 +140,9 @@ differences.
 .SH SOURCE
 .B \*9/src/cmd/diff
 .SH "SEE ALSO"
-.IM cmp (1) ,
-.IM comm (1) ,
-.IM ed (1)
+.MR cmp (1) ,
+.MR comm (1) ,
+.MR ed (1)
 .SH DIAGNOSTICS
 Exit status is the empty string
 for no differences,