stream: apply javafmt formatting to Gather.java doc example

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Author: He-Pin

docs/src/test/java/jdocs/stream/operators/flow/Gather.java

@@ -20,7 +20,6 @@
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.List;
-
 import org.apache.pekko.actor.ActorSystem;
 import org.apache.pekko.stream.javadsl.GatherCollector;
 import org.apache.pekko.stream.javadsl.Gatherer;