04-formatting.md: replace sub-heading with sub-title

Pull Request: #225

* replace sub-heading with sub-title

Author: raynamharris

_episodes/04-formatting.md

@@ -12,7 +12,7 @@ objectives:
 keypoints:
 - "Lesson episodes are stored in _episodes/dd-subject.md."
 - "Each episode's title must include a title, time estimates, motivating questions, lesson objectives, and key points."
-- "Episodes should not use sub-headings or HTML layout."
+- "Episodes should not use sub-titles or HTML layout."
 - "Code blocks can have the source, regular output, or error class."
 - "Special sections are formatted as blockquotes that open with a level-2 header and close with a class identifier."
 - "Special sections may be callouts or challenges; other styles are used by the template itself."
@@ -103,7 +103,7 @@ authors should use only:
 
 Authors should *not* use:
 
-*   sub-headings
+*   sub-titles (instead, use H2 subheadings (`##`) in the episode files)
 *   HTML layout (e.g., `div` elements).