docs/postmortems: remove wrong parentheses (ethereum#26066)

Removed parentheses in line 71 because line 80 doesn't have them either.

Author: SebastianSupreme

docs/postmortems/2021-08-22-split-postmortem.md

@@ -68,7 +68,7 @@ Since we had merged the removal of `ETH65`, if the entire network were to upgrad
 
 ## Exploit
 
-At block [13107518](https://etherscan.io/block/13107518), mined at (Aug-27-2021 12:50:07 PM +UTC), a minority chain split occurred. The discord user @AlexSSD7 notified the allcoredevs-channel on the Eth R&D discord, on Aug 27 13:09  UTC. 
+At block [13107518](https://etherscan.io/block/13107518), mined at Aug-27-2021 12:50:07 PM +UTC, a minority chain split occurred. The discord user @AlexSSD7 notified the allcoredevs-channel on the Eth R&D discord, on Aug 27 13:09  UTC. 
 
 
 At 14:09 UTC, it was confirmed that the transaction `0x1cb6fb36633d270edefc04d048145b4298e67b8aa82a9e5ec4aa1435dd770ce4` had triggered the bug, leading to a minority-split of the chain. The term 'minority split' means that the majority of miners continued to mine on the correct chain.