Correct error in attribute list

RoboFile.php

@@ -376,7 +376,8 @@ class RoboFile extends \Robo\Tasks {
                     'radius'                       => ['self::SVG_NAMESPACE' => ["feMorphology"]],
                     'refX'                         => ['self::SVG_NAMESPACE' => ["marker", "symbol"]],
                     'refY'                         => ['self::SVG_NAMESPACE' => ["marker", "symbol"]],
-                    'referrerpolicy'               => ['self::SVG_NAMESPACE' => ["a", "a"]],
+                    'referrerpolicy'               => ['self::SVG_NAMESPACE' => ["a"]],
+                    'rel'                          => ['self::SVG_NAMESPACE' => ["a"]],
                     'repeatCount'                  => ['self::SVG_NAMESPACE' => ["animate", "animateMotion", "animateTransform", "set"]],
                     'repeatDur'                    => ['self::SVG_NAMESPACE' => ["animate", "animateMotion", "animateTransform", "set"]],
                     'requiredExtensions'           => ['self::SVG_NAMESPACE' => ["a", "animate", "animateMotion", "animateTransform", "audio", "canvas", "circle", "clipPath", "discard", "ellipse", "foreignObject", "g", "iframe", "image", "line", "mask", "path", "polygon", "polyline", "rect", "set", "svg", "switch", "text", "textPath", "tspan", "unknown", "use", "video"]],