Remove mention that "item" should be interpreted JSON Schema core

The "item" term is no longer defined in JSON Schema core document (from
3cbfc8b). So remove this reference from
hyperschema document.

Author: Denis Laxalde

jsonschema-hyperschema.xml

@@ -106,7 +106,7 @@
             </t>
 
             <t>
-                The terms "schema", "instance", "property" and "item" are to be interpreted as defined in the <xref target="json-schema">JSON Schema core
+                The terms "schema", "instance" and "property" are to be interpreted as defined in the <xref target="json-schema">JSON Schema core
                 specification</xref>.
             </t>
         </section>