Add images to your Article Schema throws fatal error

Please give us a description of the issue

When I use the filter wpseo_schema_article and use the example code provided here, I see fatal error.

Steps to replicate:

1. I used the code snippet plugin to add the example code.
2. Changed the value of $attachment_id in the example. I added an image and used the ID for that image
3. Then I saved the code and activated it.
4. I checked a post where I set schema type as webpage and page type as article. I noticed fatal error in the page source that says: WPSEO_Schema_Image is not found.

Screenshot:

• I've read and understood the contribution guidelines.
• I've searched for any related issues and avoided creating a duplicate issue.

Start with the “Add images to your Article Schema” example linked in the issue and reproduce it using the reported filter, attachment ID, and Article schema settings. Check why the example refers to WPSEO_Schema_Image, then update the documented example so it no longer produces the reported fatal error and verify the generated page source.