Update Array Helper documentation

default value of the "default value" parameter has changed
in element() and elements() methods
diff --git a/system/helpers/array_helper.php b/system/helpers/array_helper.php
index 216f12e..5d02439 100644
--- a/system/helpers/array_helper.php
+++ b/system/helpers/array_helper.php
@@ -43,7 +43,7 @@
 	 * Element
 	 *
 	 * Lets you determine whether an array index is set and whether it has a value.
-	 * If the element is empty it returns FALSE (or whatever you specify as the default value.)
+	 * If the element is empty it returns NULL (or whatever you specify as the default value.)
 	 *
 	 * @param	string
 	 * @param	array