platform/x86: thinkpad_acpi: rectify length of title underline

Commit d7cbe277 ("platform/x86: thinkpad_acpi: set keyboard language")
adds information on keyboard setting to the thinkpad documentation, but
made the subsection title underline too short.

Hence, make htmldocs warns:

  Documentation/admin-guide/laptops/thinkpad-acpi.rst:1472: \
    WARNING: Title underline too short.

Rectify length of subsection title underline.
Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Link: https://lore.kernel.org/r/20210129040849.26740-1-lukas.bulwahn@gmail.comSigned-off-by: Hans de Goede <hdegoede@redhat.com>

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

@@ -1469,7 +1469,7 @@ Sysfs notes
 
 
 Setting keyboard language
--------------------
+-------------------------
 
 sysfs: keyboard_lang