hwmon: sy7636a: Fix trivial 'underline too short' warning

 Documentation/hwmon/sy7636a-hwmon.rst:4: WARNING: Title underline too short.

 Kernel driver sy7636a-hwmon
 =========================

Acked-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/all/YgTRu7iLgJMymeT5@google.com/