Fix typo in list.length= method comment

Change-Id: I35457f926446c9b77c34d7b2a967b6d9076ba055
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/169841
Auto-Submit: Karl Klose <karlklose@google.com>
Commit-Queue: Karl Klose <karlklose@google.com>
Commit-Queue: Lasse R.H. Nielsen <lrn@google.com>
Reviewed-by: Lasse R.H. Nielsen <lrn@google.com>

Author: karlklose

sdk/lib/core/list.dart

@@ -337,7 +337,7 @@ abstract class List<E> implements EfficientLengthIterable<E> {
    * Increasing the length fails if the element type does not allow `null`.
    *
    * Throws an [UnsupportedError] if the list is fixed-length or
-   * if attempting tp enlarge the list when `null` is not a valid element.
+   * if attempting to enlarge the list when `null` is not a valid element.
    */
   set length(int newLength);