Patch Blazor Layout topic example (#17755)

Author: guardrex

aspnetcore/blazor/layouts/sample_snapshot/3.x/App2.razor

@@ -3,7 +3,7 @@
         <RouteView RouteData="@routeData" DefaultLayout="@typeof(MainLayout)" />
     </Found>
     <NotFound>
-        <LayoutView Layout="typeof(MainLayout)">
+        <LayoutView Layout="@typeof(MainLayout)">
             <h1>Page not found</h1>
             <p>Sorry, there's nothing at this address.</p>
         </LayoutView>