Use path_join() function

Co-authored-by: Vicente Canales <1157901+vcanales@users.noreply.github.com>

admin/create-theme/theme-zip.php

@@ -195,7 +195,7 @@ public static function add_templates_to_zip( $zip, $export_type, $new_slug ) {
 
 			// Write the template content
 			$zip->addFromStringToTheme(
-				$template_folders['wp_template_part'] . '/' . $template->slug . '.html',
+				path_join( $template_folders['wp_template_part'], $template->slug . '.html' ),
 				$template->content
 			);