Fix typo in lib document.

Change-Id: Ia6c3b545c53a9604a3fe670ef232a6033d74c1ef
Reviewed-on: https://dart-review.googlesource.com/17223
Reviewed-by: Florian Loitsch <floitsch@google.com>

docs/newsletter/lib/lib.md

@@ -565,7 +565,7 @@ class DateTime implements Comparable<DateTime> {
   DateTime.now() : this._now();
 
   // Improved to accept RFC1123 format (and more of RFC 3399).
-  // Now errors, if the entries are out of range (like a month alue of 13).
+  // 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);