Update service_subscribers_locators.rst

According to symfony/symfony#34637, `index_by` attribute must be set to make `default_index_method` working

Author: l-vo

service_container/service_subscribers_locators.rst

@@ -526,7 +526,7 @@ attribute to the locator service defining the name of this custom method:
             # ...
 
             App\HandlerCollection:
-                arguments: [!tagged_locator { tag: 'app.handler', default_index_method: 'myOwnMethodName' }]
+                arguments: [!tagged_locator { tag: 'app.handler', index_by: 'key', default_index_method: 'myOwnMethodName' }]
 
     .. code-block:: xml
 
@@ -542,7 +542,7 @@ attribute to the locator service defining the name of this custom method:
                 <!-- ... -->
 
                 <service id="App\HandlerCollection">
-                    <argument type="tagged_locator" tag="app.handler" default-index-method="myOwnMethodName"/>
+                    <argument type="tagged_locator" tag="app.handler" index-by="key" default-index-method="myOwnMethodName"/>
                 </service>
             </services>
         </container>
@@ -555,9 +555,12 @@ attribute to the locator service defining the name of this custom method:
         return function(ContainerConfigurator $configurator) {
             $configurator->services()
                 ->set(App\HandlerCollection::class)
-                    ->args([tagged_locator('app.handler', null, 'myOwnMethodName')])
+                    ->args([tagged_locator('app.handler', 'key', 'myOwnMethodName')])
             ;
         };
+        
+Note since code should not be the place that decides how the locators are going to be used, a configuration key (`key` in the example above) must be set in order this to work. So the custom method may be called as a fallback.
+        
 
 Service Subscriber Trait
 ------------------------