formatting

book/0_background_information.html

@@ -487,7 +487,7 @@ fn main() {
 <p>Still hanging in there? Good. Don't get frustrated if the code above is
 difficult to understand. If I hadn't written it myself I would probably feel
 the same. You can always go back and read the book which explains it later.</p>
-<h3><a class="header" href="#callback-based-approaches" id="callback-based-approaches">Callback based approaches</a></h3>
+<h2><a class="header" href="#callback-based-approaches" id="callback-based-approaches">Callback based approaches</a></h2>
 <p>You probably already know what we're going to talk about in the next paragraphs
 from Javascript which I assume most know. </p>
 <blockquote>

book/print.html

@@ -529,7 +529,7 @@ fn main() {
 <p>Still hanging in there? Good. Don't get frustrated if the code above is
 difficult to understand. If I hadn't written it myself I would probably feel
 the same. You can always go back and read the book which explains it later.</p>
-<h3><a class="header" href="#callback-based-approaches" id="callback-based-approaches">Callback based approaches</a></h3>
+<h2><a class="header" href="#callback-based-approaches" id="callback-based-approaches">Callback based approaches</a></h2>
 <p>You probably already know what we're going to talk about in the next paragraphs
 from Javascript which I assume most know. </p>
 <blockquote>

src/0_background_information.md

@@ -356,7 +356,7 @@ Still hanging in there? Good. Don't get frustrated if the code above is
 difficult to understand. If I hadn't written it myself I would probably feel
 the same. You can always go back and read the book which explains it later.
 
-### Callback based approaches
+## Callback based approaches
 
 You probably already know what we're going to talk about in the next paragraphs
 from Javascript which I assume most know.