Increase default value for leadingScreensForBatching to 2.0, as 1.0 is not enough

for most network conditions / backend performance.

AsyncDisplayKit/ASCollectionView.h

@@ -102,7 +102,7 @@ NS_ASSUME_NONNULL_BEGIN
 /**
  * The number of screens left to scroll before the delegate -collectionView:beginBatchFetchingWithContext: is called.
  *
- * Defaults to one screenful.
+ * Defaults to two screenfuls.
  */
 @property (nonatomic, assign) CGFloat leadingScreensForBatching;