Correct typo in aria attributes' names

There should not be a dash before “by”:

- http://www.w3.org/TR/wai-aria/states_and_properties#aria-labelledby
- http://www.w3.org/TR/wai-aria/states_and_properties#aria-describedby

This fixes a typo I introduced in zendframework/zf2/#5989.

src/View/Helper/AbstractHelper.php

@@ -130,8 +130,8 @@ abstract class AbstractHelper extends BaseAbstractHelper
         'onvolumechange'     => true,
         'onwaiting'          => true,
         'role'               => true,
-        'aria-labelled-by'   => true,
-        'aria-described-by'  => true,
+        'aria-labelledby'    => true,
+        'aria-describedby'   => true,
         'spellcheck'         => true,
         'style'              => true,
         'tabindex'           => true,