index_custom_elasticsearch_data.md: Links to PHP API Ref

Author: adriendupuis

docs/search/extensibility/index_custom_elasticsearch_data.md

@@ -16,7 +16,7 @@ These events are called when the index is created for the content and location d
 
 You can pass the event to a subscriber which gives you access to the document that you can modify.
 
-In the following example, when an index in created for a content or a location document, the event subscriber adds a `custom_field` of the type `StringField` to the index:
+In the following example, when an index in created for a content or a location document, the event subscriber adds a `custom_field` of the type [`StringField`](../../api/php_api/php_api_reference/classes/Ibexa-Contracts-Core-Search-FieldType-StringField.html) to the index:
 
 ``` php hl_lines="19 20 21"
 <?php