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

Suggested by G. Brandon Robinson.

man/man3/venti-file.3

@@ -99,7 +99,7 @@ void	vtfileunlock(VtFile *f);
 .SH DESCRIPTION
 These routines provide a simple interface to create and
 manipulate Venti file trees (see
-.IM venti (7) ).
+.MR venti (7) ).
 .PP
 .I Vtfilecreateroot
 creates a new Venti file.
@@ -226,7 +226,7 @@ if an error is encountered.
 .I Vtfilewrite
 writes to an in-memory copy of the data blocks
 (see
-.IM venti-cache (3) )
+.MR venti-cache (3) )
 instead of writing directly to Venti.
 .I Vtfileflush
 writes all copied blocks associated with 
@@ -319,7 +319,7 @@ in the same directory block.
 .SH SOURCE
 .B \*9/src/libventi/file.c
 .SH SEE ALSO
-.IM venti-cache (3) ,
-.IM venti-conn (3) ,
-.IM venti-client (3) ,
-.IM venti (7)
+.MR venti-cache (3) ,
+.MR venti-conn (3) ,
+.MR venti-client (3) ,
+.MR venti (7)