Replace the phrase "feature set" with dialect

Author: Relequestual

jsonschema-core.xml

@@ -1168,9 +1168,9 @@
                 </t>
                 <section title='The "$schema" Keyword' anchor="keyword-schema">
                     <t>
-                        The "$schema" keyword is both used as a JSON Schema feature set identifier and
+                        The "$schema" keyword is both used as a JSON Schema dialect identifier and
                         as the identifier of a resource which is itself a JSON Schema, which describes the
-                        set of valid schemas written for this particular feature set.
+                        set of valid schemas written for this particular dialect.
                     </t>
                     <t>
                         The value of this keyword MUST be a <xref target="RFC3986">URI</xref>