Fix sentence fragment

Author: rjmill

jsonschema-core.xml

@@ -306,7 +306,7 @@
                 plain name fragment identifiers.
             </t>
             <t>
-                Defining and referencing a plain name fragment identifier
+                Defining and referencing a plain name fragment identifier are specified
                 in the <xref target="id-keyword">"$id" keyword</xref> section.
             </t>
             <t>