Fix typo in default phame config

Summary: I skipped lint because it was being angry at me.

Test Plan: ran phame with new default, was able to join blogosphere

Reviewers: epriestley, codeblock

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D4618

src/applications/phame/config/PhabricatorPhameConfigOptions.php

@@ -17,7 +17,7 @@ final class PhabricatorPhameConfigOptions
         'phame.skins',
         'list<string>',
         array(
-          'exterals/skins/',
+          'externals/skins/',
         ))
         ->setDescription(
           pht('List of directories where Phame will look for skins.')),