doc(SelectRegion): add inject service method doc (#7342)

Author: ArgoZhang

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

@@ -1,4 +1,4 @@
-@page "/select-region"
+@page "/select-region"
 @inject IStringLocalizer<SelectRegions> Localizer
 
 <h3>@Localizer["RegionTitle"]</h3>
@@ -7,6 +7,8 @@
 
 <PackageTips Name="BootstrapBlazor.Region" />
 
+<Pre>builder.Services.AddBootstrapBlazorRegionService()</Pre>
+
 <DemoBlock Title="@Localizer["RegionNormalTitle"]"
            Introduction="@Localizer["RegionNormalIntro"]"
            Name="Normal">