We previously indented them by two spaces, following a
common convention. Since the convention is fading, and
the indentation is inconvenient for copy/paste, we are
discontinuing this practice.
Closes#5440.
Emacs always uses two spaces when indenting the content of src blocks,
e.g., when exiting a `C-c '` edit-buffer. Pandoc used to indent contents
by the space-equivalent of one tab, but now always uses two spaces, too.
Closes: #5440
