Explicitly mention the @ sign in instance annotations

Chapter 18, second paragraph: change Annotations are name/value pairs that have a dot (.) as part of the name to Annotations are name/value pairs that have an at (@) and a dot (.) as part of the name Chapter 20, first sentence: change Implementations can add custom annotations of the form namespace.termname or property@namespace.termname to Implementations can add custom annotations of the form @namespace.termname or property@namespace.termname