Fix docs sample for ConversionServiceFactoryBean

Fix the reference documentation example for ConversionServiceFactoryBean
to use a `<set>` instead of a `<list>`.

Issue: SPR-10802

src/asciidoc/index.adoc

@@ -10335,9 +10335,9 @@ either of the Converter, ConverterFactory, or GenericConverter interfaces.
 <bean id="conversionService"
       class="org.springframework.context.support.ConversionServiceFactoryBean">
     <property name="converters">
-        <list>
+        <set>
             <bean class="example.MyCustomConverter"/>
-        </list>
+        </set>
     </property>
 </bean>
 ----