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>

Documentation/input/multi-touch-protocol.txt

@@ -361,7 +361,7 @@ For win8 devices with both T and C coordinates, the position mapping is
    ABS_MT_POSITION_X := T_X
    ABS_MT_POSITION_Y := T_Y
    ABS_MT_TOOL_X := C_X
-   ABS_MT_TOOL_X := C_Y
+   ABS_MT_TOOL_Y := C_Y
 
 Unfortunately, there is not enough information to specify both the touching
 ellipse and the tool ellipse, so one has to resort to approximations.  One