Fix typo

- "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."

- "Code blocks can be have the source, regular output, or error class."

- "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."

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