Inherit the correct isolation level with #snapshot

Fix a typo in `RDF::Repository#isolation_level` to inherit the correct
level when snapshot is implemented. This affects Repositories
implementing snapshots, but not overriding the default
`#isolation_level`.

lib/rdf/repository.rb

@@ -212,7 +212,7 @@ def project_graph(graph_name, &block)
     ##
     # @see RDF::Dataset#isolation_level
     def isolation_level
-      supports?(:snapshot) ? :repeatable_read : super
+      supports?(:snapshots) ? :repeatable_read : super
     end
 
     ##