Fix example URL inconsistency

In case of the GitHub example links like `<a href="https://github.com/{{ org.username }}">` with the full URL plus the variable were used, but for the Twitter example link only the `{{  author.name }}` was set as `href`.

docs/_docs/datafiles.md

@@ -143,7 +143,7 @@ author: dave
 
 {% assign author = site.data.people[page.author] %}
 <a rel="author"
-  href="{{ author.twitter }}"
+  href="https://twitter.com/{{ author.twitter }}"
   title="{{ author.name }}">
     {{ author.name }}
 </a>