hwmon: pmbus: fix table in STEF48H28 documentation

hwmon: pmbus: fix table in STEF48H28 documentation

"make htmldocs" reports build errors/warnings:

Documentation/hwmon/stef48h28.rst:45: ERROR: Missing matching underline for section title overline.
=========================================================
in1_label               "vin".
in1_input               Measured voltage. From READ_VIN register. [docutils]
Documentation/hwmon/stef48h28.rst:73: WARNING: Title underline too short.
temp2_input             Measured temperature. From READ_TEMPERATURE_2 register.
========================================================= [docutils]

Fix the table overline/underline line lengths and convert it to
a 2-column table.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Link: https://lore.kernel.org/r/20260128002200.1665941-1-rdunlap@infradead.org
Signed-off-by: Guenter Roeck <linux@roeck-us.net>