Fix singular to plural

"these keywords" for both "$anchor" and "$dynamicAnchor"

Author: handrews

jsonschema-core.xml

@@ -1467,7 +1467,7 @@
                         need for "$dynamicAnchor".
                     </t>
                     <t>
-                        If present, the value of this keyword MUST be a string and MUST conform
+                        If present, the value of these keywords MUST be a string and MUST conform
                         to the plain name fragment identifier syntax defined in section
                         <xref target="fragments" format="counter"></xref>.
                         <cref>