use the highlight tag for upgrading docs code block

site/_docs/upgrading/2-to-3.md

@@ -109,18 +109,18 @@ In Jekyll 2, any URL constructed from the `permalink:` field had a trailing slas
 
 Try adding `future: true` to your `_config.yml` file. Are they showing up now? If they are, then you were ensnared by an issue with the way Ruby parses times. Each of your posts is being read in a different timezone than you might expect and, when compared to the computer's current time, is "in the future." The fix for this is to add [a timezone offset](https://en.wikipedia.org/wiki/List_of_UTC_time_offsets) to each post (and make sure you remove `future: true` from your `_config.yml` file). If you're writing from California, for example, you would change this:
 
-```yaml
+{% highlight yaml %}
 ---
 date: 2016-02-06 19:32:10
 ---
-```
+{% endhighlight %}
 
 to this (note the offset):
 
-```yaml
+{% highlight yaml %}
 ---
 date: 2016-02-06 19:32:10 -0800
 ---
-```
+{% endhighlight %}
 
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