chore(dateInput): add missing using statements

Author: marin-bratanov

components/dateinput/overview.md

@@ -72,6 +72,9 @@ The date input provides the following features:
 >caption Example of using validation to prompt the user for certain input
 
 ````CSHTML
+@using Telerik.Blazor.Components.DateInput
+@using System.ComponentModel.DataAnnotations
+
 <EditForm Model="@person">
     <DataAnnotationsValidator />
     <ValidationSummary />