Changed display mode of summary tag

Firefox does not show arrow next to summary when is displayed as block. The right display mode for summary is list-item. See: https://github.com/rstudio/rmarkdown/issues/1483

independent-publisher-2/style.css

@@ -80,11 +80,14 @@ hgroup,
 main,
 menu,
 nav,
-section,
-summary {
+section {
 	display: block;
 }
 
+summary {
+	display: list-item;
+}
+
 audio,
 canvas,
 progress,