Fix warning with Elixir v1.18

Author: marcelotto

lib/rdf/xml/decoder/grammar/rule.ex

@@ -215,9 +215,10 @@ defmodule RDF.XML.Decoder.Grammar.Rule do
 
       defoverridable unquote(__MODULE__)
 
+      @dialyzer {:nowarn_function, element: 1}
       def element(cxt) do
         if element_cxt = element_cxt(cxt) do
-          element_cxt.element
+          Map.get(element_cxt, :element)
         end
       end
     end