[time.zone.leap.overview] Fix example (#6383)

Author: hewillk

source/time.tex

@@ -10159,7 +10159,7 @@
 \begin{example}
 \begin{codeblock}
 for (auto& l : get_tzdb().leap_seconds)
-  if (l <= 2018y/March/17d)
+  if (l <= sys_days{2018y/March/17d})
     cout << l.date() << ": " << l.value() << '\n';
 \end{codeblock}