Change ExampleRescoreBuilder ctor visibility (#50922)

The existing package private visibility of the serialization ctor isn't enough when copying
example code to oder packages or projects when bootstrapping a new project from it.

plugins/examples/rescore/src/main/java/org/elasticsearch/example/rescore/ExampleRescoreBuilder.java

@@ -64,7 +64,7 @@ public class ExampleRescoreBuilder extends RescorerBuilder<ExampleRescoreBuilder
         this.factorField = factorField;
     }
 
-    ExampleRescoreBuilder(StreamInput in) throws IOException {
+    public ExampleRescoreBuilder(StreamInput in) throws IOException {
         super(in);
         factor = in.readFloat();
         factorField = in.readOptionalString();