Fixed reference to IsHidden

`IsHidden` was removed from `AttributeDescriptor`. Needed to fix XmlDoc reference to point to `Topic.IsHidden` instead.

Author: JeremyCaney

Ignia.Topics/AttributeDescriptor.cs

@@ -120,7 +120,8 @@ public string Type {
     /// <remarks>
     ///   When attributes are displayed in the editor, they are grouped by tabs. The tabs are not predetermined, but rather set
     ///   by individual attributes. If five attributes, for instance, have a display group of "Settings", then a tab will be
-    ///   rendered called "Settings" and will list those five attributes (assuming none are set to <see cref="IsHidden"/>).
+    ///   rendered called "Settings" and will list those five attributes (assuming none are set to <see
+    ///   cref="Topic.IsHidden"/>).
     /// </remarks>
     /// <requires description="The value from the getter must be specified." exception="T:System.ArgumentNullException">
     ///   !String.IsNullOrWhiteSpace(value)