Fixes headline order in introduction

docs/book/intro.md

@@ -51,7 +51,7 @@ This decision was made for several reasons:
 > implement PSR-11, especially with zend-servicemanager. You can even use
 > factories generated by zend-di for zend-servicemanager.
 
-# Dependency Injection Containers
+## Dependency Injection Containers
 
 When your code is written in such a way that all your dependencies are injected
 into consuming objects, you might find that the simple act of wiring an object
@@ -78,7 +78,7 @@ to obtain the instances of the dependencies.
 While for the simplest use cases no configuration is needed, zend-di allows
 developers to configure how to resolve dependencies for more complex use cases.
 
-# Legacy documentation
+## Legacy documentation
 
 The primary documentation covers version 3.0 and above. We also ship
 documentation covering version 2 features, starting with a chapter covering