docs: filesystems: Reduce ext2.rst to one top-level heading

This prevents the other headings like "Options" and "Specification" from
leaking out and being listed separately in the table of contents.

Link: https://lore.kernel.org/r/20201108004045.1378676-1-j.neuschaefer@gmx.net


Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Signed-off-by: Jan Kara <jack@suse.cz>