Fixed duplicate word in comment in datetime.js (#1623)

src/datetime.js

@@ -690,7 +690,7 @@ export default class DateTime {
         `fromMillis requires a numerical input, but received a ${typeof milliseconds} with value ${milliseconds}`
       );
     } else if (milliseconds < -MAX_DATE || milliseconds > MAX_DATE) {
-      // this isn't perfect because because we can still end up out of range because of additional shifting, but it's a start
+      // this isn't perfect because we can still end up out of range because of additional shifting, but it's a start
       return DateTime.invalid("Timestamp out of range");
     } else {
       return new DateTime({