Merge pull request #232 from lektor/checkboxes

[checkboxes] Fixing incorrect example; .id -> ._id

content/docs/api/db/types/checkboxes/contents.lr

@@ -43,7 +43,7 @@ source = record.attachments.images
 
 ```html+jinja
 {% for image in this.attachments.images %}
-  {% if image.id in this.slideshow %}
+  {% if image._id in this.slideshow %}
     <img src="{{ image.thumbnail(500) }}" class="slide">
   {% endif %}
 {% endfor %}