Add a bottom table border to complete the table format and prevent
a documentation build warning.

Documentation/ABI/testing/sysfs-class-power:459: WARNING: Malformed table.
No bottom table border found.

Fixes: 1b0b6cc8 ("power: supply: add charge_behaviour attributes")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Thomas Weißschuh <linux@weissschuh.net>
Cc: Hans de Goede <hdegoede@redhat.com>
Cc: Sebastian Reichel <sre@kernel.org>
Cc: linux-pm@vger.kernel.org
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>