Revert "Fix parameter name mismatch in Playlist"

This reverts commit 65b49a49.

library/core/src/main/java/com/google/android/exoplayer2/Playlist.java

@@ -119,7 +119,7 @@ import java.util.Set;
           MediaSourceHolder previousHolder = mediaSourceHolders.get(insertionIndex - 1);
           Timeline previousTimeline = previousHolder.mediaSource.getTimeline();
           holder.reset(
-              /* firstWindowIndexInChild= */ previousHolder.firstWindowIndexInChild
+              /* firstWindowInChildIndex= */ previousHolder.firstWindowIndexInChild
                   + previousTimeline.getWindowCount());
         } else {
           holder.reset(/* firstWindowIndexInChild= */ 0);