Update sqlserver.md (#738)

Changed the misprint of Postgre to SqlServer

docs/content/user-guide/en/storage/sqlserver.md

@@ -24,7 +24,7 @@ public void ConfigureServices(IServiceCollection services)
 
     services.AddCap(x =>
     {
-        x.UsePostgreSql(opt=>{
+        x.UseSqlServer(opt=>{
             //SqlServerOptions
         }); 
         // x.UseXXX ...