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: Randy Dunlap <rdunlap@infradead.org>
Cc: Nitin Joshi <njoshi1@lenovo.com>
Cc: Sugumaran <slacshiminar@lenovo.com>
Cc: Bastien Nocera <bnocera@redhat.com>
Cc: Mark Pearson <markpearson@lenovo.com>
Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Darren Hart <dvhart@infradead.org>
Cc: Henrique de Moraes Holschuh <ibm-acpi@hmh.eng.br>
Cc: ibm-acpi-devel@lists.sourceforge.net
Cc: platform-driver-x86@vger.kernel.org
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>