hwmon: (pmbus/q54sj108a2) Correct title underline length

Commit b722d7b9e4da ("hwmon: (pmbus) Driver for Delta power supplies
Q54SJ108A2") provides new documentation for DELTA Q54SJ108A2NC* drivers,
but the title underline was too short.

make htmldocs warns:

  Documentation/hwmon/q54sj108a2.rst:4: WARNING: Title underline too short.

Adjust the title underline to the correct length.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Link: https://lore.kernel.org/r/20201207152658.32444-1-lukas.bulwahn@gmail.com
[groeck: Adjust subject]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>