add missing comma

Author: aristath

field/class-kirki-field-gradient.php

@@ -40,7 +40,7 @@ protected function set_choices() {
 			'alpha'   => true,
 			'preview' => array(
 				'width'  => '100%',
-				'height' => '60px'
+				'height' => '60px',
 			),
 		);
 		$this->choices = wp_parse_args( $this->choices, $defaults );