doc(Select): add generic type documentation (#6072)

Author: ArgoZhang

src/BootstrapBlazor.Server/Components/Samples/SelectGenerics.razor

@@ -342,7 +342,7 @@
             <SelectGeneric TValue="string" Items="Items">
                 <DisplayTemplate>
                     <i class="fa-solid fa-flag"></i>
-                    <span>@context.Text</span>
+                    <span>@context?.Text</span>
                 </DisplayTemplate>
             </SelectGeneric>
         </div>