Fix calendar ref which was causing issues

This reference to the calendar function causes issues in many cases.
If you feed the function an epoch date (ms), it would spit out the calendar date of the current time.

This simple change fixes that issue and many others.

docs/plugin/calendar.md

@@ -11,7 +11,7 @@ var calendar = require("dayjs/plugin/calendar");
 
 dayjs.extend(calendar);
 
-dayjs().calendar(dayjs("2008-01-01"));
+dayjs("2008-01-01").calendar();
 dayjs().calendar(null, {
   sameDay: "[Today at] h:mm A", // The same day ( Today at 2:30 AM )
   nextDay: "[Tomorrow at] h:mm A", // The next day ( Tomorrow at 2:30 AM )