Add missing SearchRoot assignment in example (dotnet#9573)

Author: SiavasFiroozbakht

xml/System.DirectoryServices/DirectorySearcher.xml

@@ -601,7 +601,8 @@ public class Example
     public static void Main()
     {
         DirectoryEntry group = new DirectoryEntry("LDAP://CN=MyGroup", …);        
-        DirectorySearcher src = new DirectorySearcher("(&(objectClass=user)(objectCategory=Person))");          
+        DirectorySearcher src = new DirectorySearcher("(&(objectClass=user)(objectCategory=Person))");      
+        src.SearchRoot = group;
         src.AttributeScopeQuery = "member";  
         src.PropertiesToLoad.Add("sn");
         src.PropertiesToLoad.Add("givenName");