Refer to the "default" meta-schema

Now that the core spec explicitly defines this as the default,
let's call it that.  Because calling it the "core and validation"
meta-schema is annoying.

Author: handrews

jsonschema-validation.xml

@@ -176,8 +176,8 @@
 
         <section title="Meta-Schema" anchor="meta-schema">
             <t>
-                The current URI for the JSON Schema Validation meta-schema is
-                <eref target="http://json-schema.org/draft/2019-08/schema#"/>.
+                The current URI for the default JSON Schema meta-schema is
+                <eref target="http://json-schema.org/draft/2019-08/schema"/>.
                 For schema author convenience, this meta-schema describes all vocabularies
                 defined in this specification and the JSON Schema Core specification,
                 as well as two former keywords which are reserved for a transitional period.