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

Suggested by G. Brandon Robinson.

man/man1/venti.1

@@ -40,7 +40,7 @@ read, write, copy \- simple Venti clients
 .SH DESCRIPTION
 Venti is a SHA1-addressed block storage server.
 See 
-.IM venti (7)
+.MR venti (7)
 for a full introduction.
 .PP
 .I Read
@@ -167,13 +167,13 @@ messages send/received.
 .SH SOURCE
 .B \*9/src/cmd/venti
 .SH SEE ALSO
-.IM vac (1) ,
-.IM venti (3) ,
-.IM vacfs (4) ,
-.IM venti (7) ,
-.IM vbackup (8) ,
-.IM venti (8) ,
-.IM venti-fmt (8)
+.MR vac (1) ,
+.MR venti (3) ,
+.MR vacfs (4) ,
+.MR venti (7) ,
+.MR vbackup (8) ,
+.MR venti (8) ,
+.MR venti-fmt (8)
 .SH BUGS
 There should be programs to read and write
 venti files and directories.