Input: fix the input_event struct documentation

Fix the datatype of the value field of the input_event struct which is
signed instead of unsigned.

Signed-off-by: Maud Spierings <maud_spierings@hotmail.com>
Link: https://lore.kernel.org/r/20241110-fix_input_doc-v1-1-745d5f908e61@hotmail.com


Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>