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

Suggested by G. Brandon Robinson.

man/man1/hist.1

@@ -41,7 +41,7 @@ option enables verbose debugging printout.
 The 
 .B -d
 option causes
-.IM diff (1)
+.MR diff (1)
 .B -c
 to be run for each adjacent pair of dump files, while
 .B -b
@@ -73,11 +73,11 @@ by convention, root of dump file system
 .SH SOURCE
 .B \*9/src/cmd/hist.c
 .SH SEE ALSO
-.IM yesterday (1) ,
-.IM vbackup (8)
+.MR yesterday (1) ,
+.MR vbackup (8)
 .SH BUGS
 Should be called
 .IR history ,
 but
 that name is taken by
-.IM sh (1) .
+.MR sh (1) .