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

Suggested by G. Brandon Robinson.

man/man3/sleep.3

@@ -27,7 +27,7 @@ Sleep returns \-1 if interrupted, 0 otherwise.
 causes an
 .B alarm
 note (see
-.IM notify (3) )
+.MR notify (3) )
 to be sent to the invoking process after the number of milliseconds
 given by the argument.
 Successive calls to
@@ -39,7 +39,7 @@ the alarm clock.
 .SH SOURCE
 .B \*9/src/lib9/sleep.c
 .SH SEE ALSO
-.IM intro (3)
+.MR intro (3)
 .SH DIAGNOSTICS
 These functions set
 .IR errstr .
@@ -53,4 +53,4 @@ are preprocessor macros defined as
 and
 .IR p9alarm ;
 see
-.IM intro (3) .
+.MR intro (3) .