Fixed formatting and numbering

I tried to update the indenting for a more consistent look. The main ideas are: - Numberings are indented to the same column (column 3) - other non-example text is indented to the same column as text in numbered sections (column 5) - each lesson ends with two blank lines - lesson headings have the same indentation I also fixed numbering errors in lessons 13 and 15 and changed lesson headings to uppercase.