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

Suggested by G. Brandon Robinson.

man/man3/read.3

@@ -65,7 +65,7 @@ if this is not the same as requested.
 and
 .I Pwrite
 equivalent to a
-.IM seek (3)
+.MR seek (3)
 to
 .I offset
 followed by a
@@ -83,10 +83,10 @@ without interference.
 .SH SOURCE
 .B \*9/src/lib9/readn.c
 .SH SEE ALSO
-.IM intro (3) ,
+.MR intro (3) ,
 .IR open (3), 
-.IM dup (3) ,
-.IM pipe (3)
+.MR dup (3) ,
+.MR pipe (3)
 .SH DIAGNOSTICS
 These functions set
 .IR errstr .