Fix formatting in class-markdown.php

Corrected the syntax for incrementing the list_index in class-markdown.php. The updated code ensures that the variable is incremented in a clear and well-recognized format, improving code readability and maintaining consistency across the project.

includes/apple-exporter/class-markdown.php

@@ -205,7 +205,7 @@ private function parse_node_list_item( $node ) {
 		if ( 'ol' === $this->list_mode ) {
 			return sprintf(
 				'%d. %s',
-				$this->list_index ++,
+				$this->list_index++,
 				$text
 			);
 		}