docs(serialization): update transform usage

* use transform correctly like the example https://github.com/nestjs/nest/blob/25a92ccce101e3339b5c88a3615f2984b0ef5556/sample/21-serializer/src/entities/user.entity.ts#L17-L18

Author: marcjulian

content/techniques/serialization.md

@@ -74,7 +74,7 @@ get fullName(): string {
 You can perform additional data transformation using the `@Transform()` decorator. For example, the following construct returns the name property of the `RoleEntity` instead of returning the whole object.
 
 ```typescript
-@Transform(role => role.name)
+@Transform(({ value }) => value.name)
 role: RoleEntity;
 ```