[docs] fixing @GraphQLType link in unions docs

Author: Dariusz Kuc

website/docs/schema-generator/writing-schemas/unions.md

@@ -51,7 +51,7 @@ type Query {
 
 ## `@GraphQLUnion`
 :::note
-Instead of this custom annotation, the [@GraphQLType](../customizing-schemas/custom-type-reference) annotation may be a better option
+Instead of this custom annotation, the [@GraphQLType](../customizing-schemas/custom-type-reference.md) annotation may be a better option
 :::
 
 The downside to marker interface unions is that you can not edit classes included in dependencies to implement new schema unions.
@@ -110,4 +110,4 @@ However since we can not enforce the type checks anymore, you must use `Any` as
 Since this union is defined with an added annotation it is not currently possible to add directives directly to this union definition.
 You will have to modify the type with [schema generator hooks](../customizing-schemas/generator-config.md).
 
-[@GraphQLType](../customizing-schemas/custom-type-reference) annotation can be used as a workaround to this issue.
+[@GraphQLType](../customizing-schemas/custom-type-reference.md) annotation can be used as a workaround to this issue.