chore(common): add missed namespace for icons

Author: marin-bratanov

common-features/font-icons.md

@@ -19,6 +19,7 @@ You can find the full list of available icons in the [Kendo UI Web Font Icons Li
 >caption How to use a built-in font icon class on a component's Icon property
 
 ````CSHTML
+@using Telerik.Blazor
 @using Telerik.Blazor.Components.Button
 
 <TelerikButton Icon="@IconName.Filter">I show the Filter icon</TelerikButton>