minor #18418 [DependencyInjection] Fix xml example for defaut-index-name for tagged service provider (alexander-schranz)

This PR was merged into the 5.4 branch.

Discussion
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[DependencyInjection] Fix xml example for defaut-index-name for tagged service provider

Comment say it should be `getIndex` instead of `someFunctionName`.

Commits
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57c18a8 Fix xml example for defaut-index-name for tagged service provider

Author: javiereguiluz

service_container/tags.rst

@@ -997,7 +997,7 @@ with the ``default_index_method`` attribute on the tagged argument:
                     <!-- use getIndex() instead of getDefaultIndexName() -->
                     <argument type="tagged_iterator"
                         tag="app.handler"
-                        default-index-method="someFunctionName"
+                        default-index-method="getIndex"
                     />
                 </service>
             </services>