Fixed one-word typo in _config.yml (Non-Urgent) (alshedivat#1288)

Author: DouglasKrouth

_config.yml

@@ -154,7 +154,7 @@ disqus_shortname: al-folio # put your disqus shortname
 # https://help.disqus.com/en/articles/1717111-what-s-a-shortname
 
 # External sources.
-# If you have blog posts published on medium.com or other exteranl sources,
+# If you have blog posts published on medium.com or other external sources,
 # you can display them in your blog by adding a link to the RSS feed.
 external_sources:
   - name: medium.com