Input: fix typo in MT documentation

Section "Event Computation" had this:
    ...
    ABS_MT_TOOL_X := C_X
    ABS_MT_TOOL_X := C_Y

Replace the second ABS_MT_TOOL_X with ABS_MT_TOOL_Y.

Signed-off-by: Daniel Martin <consume.noise@gmail.com>
Acked-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>