Fix a typo.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=138871293

library/src/main/java/com/google/android/exoplayer2/Timeline.java

@@ -197,8 +197,8 @@ public abstract class Timeline {
     public long presentationStartTimeMs;
 
     /**
-     * The windows start time in milliseconds since the epoch, or {@link C#TIME_UNSET} if unknown or
-     * not applicable. For informational purposes only.
+     * The window's start time in milliseconds since the epoch, or {@link C#TIME_UNSET} if unknown
+     * or not applicable. For informational purposes only.
      */
     public long windowStartTimeMs;