Protocol refers to modified on AnnotationCollection, but not mentioned in Model

[azaroth42]

In paragraph 4 of the Annotation Page section, we say:

The description of the Collection SHOULD include both the total and modified properties.

modified was added due to Takeshi's concerns about the dynamic page sequence changing and hence crawlers would miss newly added entries. But we didn't go back and describe it in the model.

Propose an errata to add modified to the table of properties for AnnotationCollection.