Update reliability.rst

[TdesTouches]

In the frame (just under the change I made) and in the exercises, the parity bit takes the number of bit set to 0 into account.

[kaskales]

the parity bit takes the number of bit set to 0 into account.

This seems to contradict the definition of bit parity;
"A parity bit, or check bit, is a bit added to a string of binary code to ensure that the total number of 1-bits in the string is even or odd" .

The original definition seems to be correctly written in terms of the number of bits set to 1.