Fix syntax typo in attributes reference (doctrine#9513)

Curly brackets are the annotation way of declaring array. Probably a
mistake while copy pasting some examples from annotations.

docs/en/reference/attributes-reference.rst

@@ -570,7 +570,7 @@ Example with partial indexes:
     <?php
     use Doctrine\ORM\Mapping\Index;
 
-    #[Index(name: "search_idx", columns: {"category"},
+    #[Index(name: "search_idx", columns: ["category"],
         options: [
             "where": "((category IS NOT NULL))"
         ]