Add data model reference to vocabularies

Author: handrews

jsonschema-core.xml

@@ -190,7 +190,8 @@
                     <t>
                         Whitespace and formatting concerns, including different lexical
                         representations of numbers that are equal within the data model, are thus
-                        outside the scope of JSON Schema.  JSON Schema vocabularies that wish
+                        outside the scope of JSON Schema.  JSON Schema
+                        <xref target="vocabulary">vocabularies</xref> that wish
                         to work with such differences in lexical representations SHOULD define
                         keywords to precisely interpret formatted strings within the data model
                         rather than relying on having the original JSON representation Unicode