parent -> parent_root

Co-authored-by: g11tech <develop@g11tech.io>

Author: unnawut

docs/client/containers.md

@@ -43,7 +43,7 @@ class State(Container):
 class Block(Container):
     slot: uint64
     proposer_index: uin64
-    parent: Bytes32
+    parent_root: Bytes32
     state_root: Bytes32
     votes: List[Vote, VALIDATOR_REGISTRY_LIMIT]
 ```