Make string in fallback method translatable

Author: pattonwebz

class-wp-bootstrap-navwalker.php

@@ -365,7 +365,7 @@ public static function fallback( $args ) {
 				if ( $menu_class ) {
 					$fallback_output .= ' class="' . esc_attr( $menu_class ) . '"'; }
 				$fallback_output .= '>';
-				$fallback_output .= '<li><a href="' . esc_url( admin_url( 'nav-menus.php' ) ) . '" title="">' . esc_attr( 'Add a menu', '' ) . '</a></li>';
+				$fallback_output .= '<li><a href="' . esc_url( admin_url( 'nav-menus.php' ) ) . '" title="' . esc_attr__( 'Add a menu', 'wp-bootstrap-navwalker' ) . '">' . esc_html__( 'Add a menu', 'wp-bootstrap-navwalker' ) . '</a></li>';
 				$fallback_output .= '</ul>';
 				if ( $container ) {
 					$fallback_output .= '</' . esc_attr( $container ) . '>';