tweak to more closely match existing wording

Author: gregsdennis

jsonschema-core.xml

@@ -639,7 +639,8 @@
                 <section title="Handling of unrecognized or unsupported keywords" anchor="unrecognized">
                     <t>
                         Implementations MUST refuse to process schemas which contain
-                        unrecognized or unsupported keywords.
+                        keywords which they do not recognize or which they recognize
+                        but do not support.
                     </t>
                 </section>
             </section>