fix(specs/proposals): `getNextBlockNumber` -> `nextBlockNumber`

Sherlock issues:
- https://github.com/sherlock-audit/2023-01-optimism-judging/blob/b7921698c1c537714c5f2021aa46ce0a1935ba39/1-specs-findings/1-processed/1-true/challenger-not-proposer/169.md#L1 (already fixed)
- https://github.com/sherlock-audit/2023-01-optimism-judging/blob/b7921698c1c537714c5f2021aa46ce0a1935ba39/1-specs-findings/1-processed/1-true/oracle-interface/170.md#L1

Closing: DEVRL-845

specs/proposals.md

@@ -207,7 +207,7 @@ function deleteL2Outputs(uint256 _l2OutputIndex) external
 /**
  * @notice Computes the block number of the next L2 block that needs to be checkpointed.
  */
-function getNextBlockNumber() public view returns (uint256)
+function nextBlockNumber() public view returns (uint256)
 ```
 
 ### Configuration