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

Suggested by G. Brandon Robinson.

man/man3/ctime.3

@@ -26,7 +26,7 @@ long	tm2sec(Tm *tm)
 converts a time
 .I clock
 such as returned by
-.IM time (3)
+.MR time (3)
 into
 .SM ASCII
 (sic)
@@ -85,8 +85,8 @@ is not
 .br
 .B \*9/src/lib9/tm2sec.c
 .SH "SEE ALSO"
-.IM date (1) ,
-.IM time (3)
+.MR date (1) ,
+.MR time (3)
 .SH BUGS
 The return values point to static data
 whose content is overwritten by each call.
@@ -112,4 +112,4 @@ are preprocessor macros defined as
 and
 .IR p9tm2sec ;
 see
-.IM intro (3) .
+.MR intro (3) .