Update src/engine/specification.md

Author: djrtwo

src/engine/specification.md

@@ -226,7 +226,7 @@ The payload build process is specified as follows:
 
 #### Specification
 
-1. Client software **MUST** validate `blockHash` value as being equivalent to `Keccak256(RLP(ExecutionBlockHeader))`, where `ExecutionBlockHeader` is an object having a PoW block header structure. Fields of this object are set to the corresponding payload values and constant values according to the Block structure section of [EIP-3675](https://eips.ethereum.org/EIPS/eip-3675#block-structure), extended with the corresponding section of [EIP-4399](https://eips.ethereum.org/EIPS/eip-4399#block-structure). Client software **MUST** run this validation in all cases even if this branch or any other branches of the block tree are in an active sync process.
+1. Client software **MUST** validate `blockHash` value as being equivalent to `Keccak256(RLP(ExecutionBlockHeader))`, where `ExecutionBlockHeader` is the execution layer block header (the former PoW block header structure). Fields of this object are set to the corresponding payload values and constant values according to the Block structure section of [EIP-3675](https://eips.ethereum.org/EIPS/eip-3675#block-structure), extended with the corresponding section of [EIP-4399](https://eips.ethereum.org/EIPS/eip-4399#block-structure). Client software **MUST** run this validation in all cases even if this branch or any other branches of the block tree are in an active sync process.
 
 1. Client software **MAY** initiate a sync process if requisite data for payload validation is missing. Sync process is specified in the [Sync](#sync) section.