message: nit: Fix wording of `withdrawals.md` spec
diff: @@ -69,7 +69,7 @@ This is a very simple contract that stores the a hash of the withdrawal data.
  prevent it from being re-proven. Note that the withdrawal can be proven more than once if the corresponding
  output root changes.
  1. After the withdrawal is proven, it enters a 7 day challenge period, allowing time for other network participants
  - to challenge its integrity.
  + to challenge the integrity of the corresponding output root.
  1. Once the challenge period has passed, a relayer submits the withdrawal transaction once again to the
  `OptimismPortal` contract. Again, the relayer need not be the same entity which initiated the withdrawal on L2.
  1. The `OptimismPortal` contract receives the withdrawal transaction data and verifies that the withdrawal has
  