Remove Singleton recommendation for plugin factories.

The ChangeOperatorFactory and ChangeHasOperandFactory are generally not
going to be Singletons, so remove that recommendation from the samples.

Change-Id: I96ad4640d36f83155495ee5211d05c162c103cc5
diff --git a/Documentation/dev-plugins.txt b/Documentation/dev-plugins.txt
index 1d10025..1e54cbf 100644
--- a/Documentation/dev-plugins.txt
+++ b/Documentation/dev-plugins.txt
@@ -718,7 +718,6 @@
 
 [source,java]
 ----
-@Singleton
 public class SampleOperator
     implements ChangeQueryBuilder.ChangeOperatorFactory {
   public static class MyPredicate extends OperatorChangePredicate<ChangeData> {
@@ -751,7 +750,6 @@
 new `has:sample_pluginName` operand is shown below:
 
 ====
-  @Singleton
   public class SampleHasOperand implements ChangeHasOperandFactory {
     public static class Module extends AbstractModule {
       @Override