change slot_uri of dlthing:meta_type to rdf:type

Without this, shacl exported by linkml does not contain any
reference to the types of classes for nodes, i.e. no rdf:type
as predicate, only dlthing:meta_type as predicate. External
applications have no understanding of dlthing:meta_type and want
to query triples in a graph store based on widely used standards
such as rdf:type.

src/thing/unreleased.yaml

@@ -190,7 +190,7 @@ slots:
     range: string
 
   meta_type:
-    slot_uri: dlthing:meta_type
+    slot_uri: rdf:type
     designates_type: true
     description: >-
       Type designator of a metadata object for validation and schema structure