docs: fix broken URL link

webonyx/graphql-php have changed the url in v15.0.0

Author: docmg

website/docs/custom-types.mdx

@@ -57,7 +57,7 @@ In order to create a custom output type, you need to:
 1. Design a class that extends `GraphQL\Type\Definition\ObjectType`.
 2. Register this class in the GraphQL schema.
 
-You'll find more details on the [Webonyx documentation](https://webonyx.github.io/graphql-php/type-system/object-types/).
+You'll find more details on the [Webonyx documentation](https://webonyx.github.io/graphql-php/type-definitions/object-types/).
 
 ---