Documentation: laptops: thinkpad-acpi: fix underline length build warning

Fix underline length build warning in thinkpad-acpi.rst documentation:

Documentation/admin-guide/laptops/thinkpad-acpi.rst:1437: WARNING: Title underline too short.
DYTC Lapmode sensor
------------------

Fixes: acf7f4a5 ("platform/x86: thinkpad_acpi: lap or desk mode interface")
Signed-off-by: NRandy Dunlap <rdunlap@infradead.org>
Link: https://lore.kernel.org/r/7b2ecef9-dfb7-808a-7c05-4e4f44b363c4@infradead.orgSigned-off-by: NJonathan Corbet <corbet@lwn.net>

Documentation/admin-guide/laptops/thinkpad-acpi.rst

@@ -1434,7 +1434,7 @@ on the feature, restricting the viewing angles.
 
 
 DYTC Lapmode sensor
-------------------
+-------------------
 
 sysfs: dytc_lapmode