symbol_attribute_type.md: Link to PHP API Ref

Author: adriendupuis

docs/pim/attributes/symbol_attribute_type.md

@@ -110,7 +110,7 @@ Certain formats, such as the International Standard Book Number (ISBN-10) and th
 
 To validate checksum of symbol:
 
-1\. Create a class implementing the `\Ibexa\Contracts\ProductCatalogSymbolAttribute\Value\ChecksumInterface` interface.
+1\. Create a class implementing the [`\Ibexa\Contracts\ProductCatalogSymbolAttribute\Value\ChecksumInterface`](/api/php_api/php_api_reference/classes/Ibexa-Contracts-ProductCatalogSymbolAttribute-Value-ChecksumInterface.html) interface.
 
 2\. Register the class as a service using the `ibexa.product_catalog.attribute.symbol.checksum` tag and specify the format identifier using the `format` attribute.
 
@@ -135,4 +135,4 @@ The format attribute (`my_format`) is the identifier used under the `ibexa_produ
 
 You can use `SymbolAttribute` Search Criterion to find products by symbol attribute:
 
-For more information, see [SymbolAttribute Criterion](symbolattribute_criterion.md).
+For more information, see [SymbolAttribute Criterion](symbolattribute_criterion.md).