Fix _container declaration in snippet (#36518)

Author: meiranzheng

aspnetcore/fundamentals/middleware/extensibility-third-party-container/samples/2.x/SampleApp/Startup.cs

@@ -14,9 +14,9 @@ namespace MiddlewareExtensibilitySample
 {
     public class Startup
     {
+        #region snippet1
         private Container _container = new Container();
 
-        #region snippet1
         public void ConfigureServices(IServiceCollection services)
         {
             services.AddMvc().SetCompatibilityVersion(CompatibilityVersion.Version_2_2);

aspnetcore/fundamentals/middleware/extensibility-third-party-container/samples/3.x/SampleApp/Startup.cs

@@ -14,9 +14,9 @@ namespace MiddlewareExtensibilitySample
 {
     public class Startup
     {
+        #region snippet1
         private Container _container = new Container();
 
-        #region snippet1
         public void ConfigureServices(IServiceCollection services)
         {
             services.AddRazorPages();