Documentation/UsersManual/index.rst: code reformated

extension/Documentation/UsersManual/Index.rst

@@ -1024,11 +1024,13 @@ Nesting of levels
 Levels can be nested by using curly brackets::
 
    10.sql = SELECT 'hello world'
+
    20 {
          sql = SELECT 'a new query'
          head = <h1>
          tail = </h1>
    }
+
    30 {
          sql = SELECT 'a third query'
          head = <h1>