[css-box] fix ex.12 'margin-bottom' -> 'margin'

There's en error in example 12.
`margin-bottom: 2em 0 1em 0`

Author: retrazil

css-box/Overview.html

@@ -1668,7 +1668,7 @@ <h2 class="heading settled" data-level="13" id="collapsing-margins"><span class=
    <div class="example" id="example-5697adb2">
     <a class="self-link" href="#example-5697adb2"></a> 
     <p>For example, in the following fragment with the given style rules: </p>
-<pre>p { display: block; margin-bottom: 2em 0 1em 0 }div { display: block; margin: 2.5em 0 }
+<pre>p { display: block; margin: 2em 0 1em 0 }div { display: block; margin: 2.5em 0 }
 ...
 &lt;p>First paragraph&lt;/p>
 &lt;div>
@@ -5429,4 +5429,4 @@ <h2 class="no-num no-ref heading settled" id="issues-index"><span class="content
             }
 
         });
-        </script>
+        </script>