Core: Cite definition at first usage of URI in the section

Author: awwright

jsonschema-core.xml

@@ -530,7 +530,7 @@
         <section title="Base URI and Dereferencing">
             <t>
                 To differentiate between schemas in a vast ecosystem, schemas are
-                identified by URI, and can embed references to other schemas by specifying their URI.
+                identified by <xref target="RFC3986">URI</xref>, and can embed references to other schemas by specifying their URI.
             </t>
 
             <section title="Initial Base URI">