Fix error in cluster spec

It's "Masters reply to slave vote request" section.

````
* Slave `currentEpoch` is 3
* Slave tries to be elected with epoch 4 (3+1), master replies with an ok with `currentEpoch` 5, however the reply is delayed.
* Slave tries to be elected again, with epoch 5 (4+1), the delayed reply reaches the master with `currentEpoch` 5, and is accepted as valid.
````

Delayed reply from master will be reached to slave, not master.