ext4: docs: Take out unneeded escaping

The new file Documentation/orphan/ext4.rst escapes underscores ("\_")
throughout.  However, RST doesn't actually require that, so the escaping
only succeeds in making the document less readable.  Remove the unneeded
escapes.

Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20210902220854.198850-3-corbet@lwn.net
Signed-off-by: Jonathan Corbet <corbet@lwn.net>