doc: Chromebook Coral: fix build warnings

Use valid restructured text to avoid warnings like

WARNING: Title underline too short.
WARNING: Block quote ends without a blank line; unexpected unindent.

when building with `make htmldocs`.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>