fix default content type ref

Author: picnixz

Doc/library/http.server.rst

@@ -437,7 +437,7 @@ provides three different variants:
       The default implementation looks the file's extension up in
       :attr:`extensions_map`, falling back to
       :func:`mimetypes.guess_file_type` and then to
-      :attr:`default_content_type`.
+      :attr:`'application/octet-stream'`.
 
       .. versionchanged:: 3.13
          Add :func:`mimetypes.guess_file_type` as a fallback.