verifier.py: fix typo

Author: SomberNight

electrum/verifier.py

@@ -151,7 +151,7 @@ def hash_merkle_root(cls, merkle_branch: Sequence[str], tx_hash: str, leaf_pos_i
         index = leaf_pos_in_tree
         for item in merkle_branch_bytes:
             if len(item) != 32:
-                raise MerkleVerificationFailure('all merkle branch items have to 32 bytes long')
+                raise MerkleVerificationFailure('all merkle branch items have to be 32 bytes long')
             inner_node = (item + h) if (index & 1) else (h + item)
             cls._raise_if_valid_tx(inner_node.hex())
             h = sha256d(inner_node)