examples/trickle-library: correct probability expression

Documented intent is to update with probability 1/NEW_TOKEN_PROB where
NEW_TOKEN_PROB is 0x80.  The current implementation updates with probability
1/2.  Update NEW_TOKEN_PROB and the expression to keep existing behavior
while correcting the calculation.

Signed-off-by: Peter A. Bigot <pab@pabigot.com>
Merge pull request #270 from pabigot/push/trickle-example

examples/trickle-library: correct probability expression