Update EIPS/eip-7709.md

Co-authored-by: Guillaume Ballet <[email protected]>

EIPS/eip-7709.md

@@ -27,7 +27,7 @@ The `BLOCKHASH (0x40)` opcode currently assumes that the client has knowledge of
 | `HISTORY_STORAGE_ADDRESS` | TBD    |
 | `BLOCKHASH_SERVE_WINDOW`  | `256`  |
 
-The `BLOCKHASH` opcode semantics remains the same as before. From the `fork_block` (first block where `block.timestamp >= FORK_TIMESTAMP`), the `BLOCKHASH` instruction should be updated to resolve block hash in the following manner:
+The `BLOCKHASH` opcode semantics remains the same as before. From the `fork_block` (defined as `fork_block.timestamp >= FORK_TIMESTAMP and fork_block.parent.timestamp < FORK_TIMESTAMP`), the `BLOCKHASH` instruction should be updated to resolve block hash in the following manner:
 
 ```python
 def resolve_blockhash(block: Block, state: State, arg: uint64):