Fix clippy issues.

src/context/footnote_definition.rs

@@ -39,7 +39,7 @@ render!(
         let contents = {
             let mut ret = Vec::new();
             for obj in original.contents.iter() {
-                ret.push(obj.into_render_ast_node(render_context.clone())?);
+                ret.push(obj.as_render_ast_node(render_context.clone())?);
             }
             ret
         };