Fix invalid XML

Author: Ben Hutton

jsonschema-core.xml

@@ -1851,7 +1851,7 @@
                 <section title="Differing and Default Dialects">
                     <t>
                         In addition to the rules defining the use of the "$schema" keyword
-                        <target='The "$schema" Keyword' format="counter"></xref>,
+                        <xref target='The "$schema" Keyword' format="counter"></xref>,
                         if multiple schema resources are present in a single document, then
                         schema resources which do not have a "$schema" keyword in their root
                         schema object MUST be processed as if "$schema" were present with the