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
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Fixes: acf7f4a5

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