Fix payout time for staking rewards (#255)

Signed-off-by: Dengjianping <djptux@gmail.com>

docs/manta-atlantic/Staking/02-Rewards.md

@@ -4,4 +4,4 @@ Staking rewards work the following way:
 2. This reward is allocated for later payout in the following way
     - 10% of this reward go to the collator that produced it to compensate for collator running expenses
     - The remaining 90% is split proportionally to MANTA staked between everyone who delegated to this collator (this includes a collator’s self-bond)
-3. Three *rounds* later ( max. 18 hours ), the rewards accumulated in this way are paid out automatically, directly to the addresses of the collator and every delegator
+3. Two *rounds* later ( max. 12 hours ), the rewards accumulated in this way are paid out automatically, directly to the addresses of the collator and every delegator