Fix obsolete comment change in libs document.

"DateTime.parse taking optional arg".

Change-Id: I67779495a946789ba71ff1dffcdb8089a14f8073
Reviewed-on: https://dart-review.googlesource.com/18182
Reviewed-by: Florian Loitsch <floitsch@google.com>

docs/newsletter/lib/lib.md

@@ -566,7 +566,6 @@ class DateTime implements Comparable<DateTime> {
 
   // Improved to accept RFC1123 format (and more of RFC 3399).
   // Now errors, if the entries are out of range (like a month value of 13).
-  // Now has an `onError` optional named argument.
   static DateTime parse(String formattedString);
 
   /**