manual_episode_order.html: fix typo in a comment

Author: maxim-belkin

_includes/manual_episode_order.html

@@ -35,7 +35,7 @@
        as 'url' and 'title' and that are used in
        'episode_navbar.html'.
 
-    When episode order is specified manualy, the 'lesson_episodes'
+    When episode order is specified manually, the 'lesson_episodes'
     variable contains a list of episode names ("slugs", to be precise;
     "slug" is the episode name without '.md').  Therefore, when we
     iterate over 'lesson_episodes' (in syllabus.html and navbar.html) ,