Fix typo in OutputTimeFn

----Release Notes----

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=112480742

sdk/src/main/java/com/google/cloud/dataflow/sdk/transforms/windowing/OutputTimeFn.java

@@ -111,7 +111,7 @@ private OutputTimeFn() { }
    *
    * <p>For many other {@link OutputTimeFn} implementations, such as taking the earliest or latest
    * timestamp, this will be the same as {@link #combine combine()}. Defaults for this
-   * case are provided by {@link Default}.
+   * case are provided by {@link Defaults}.
    */
   public abstract Instant merge(W intoWindow, Iterable<? extends Instant> mergingTimestamps);