fix missing link content

Author: karenetheridge

jsonschema-core.xml

@@ -457,7 +457,7 @@
 
         <section title="Fragment Identifiers" anchor="fragments">
             <t>
-                In accordance with section 3.1 of <xref target="RFC6839"></xref>,
+                In accordance with section 3.1 of <xref target="RFC6839">RFC 6839</xref>,
                 the syntax and semantics of fragment identifiers specified for
                 any +json media type SHOULD be as specified for "application/json".
                 (At publication of this document, there is no fragment identification