Better phrasing / grammar from review feedback.

Co-authored-by: Jason Desrosiers <jdesrosi@gmail.com>

Author: handrews

jsonschema-core.xml

@@ -350,7 +350,7 @@
                         and its companions, are free to define other behaviors as well.
                     </t>
                     <t>
-                        A JSON Schema MAY contain properties which are not, or are not recognized as, schema keywords.
+                        A JSON Schema MAY contain properties which are not schema keywords or are not recognized as schema keywords.
                         The behavior of such keywords is governed by section
                         <xref target="unrecognized" format="counter"></xref>.
                     </t>