Update qip-0010 to include 7b spacer

qip-0010.md

@@ -45,14 +45,18 @@ destination chain information, which this specification intends to address.
 ## Specification
 ### Transaction Identifiers
 Transaction identifiers will consist of a permuted transaction hash.
-Specifically, the first two bytes of the hash will be overwritten with the
+Specifically, the first four bytes of the hash will be bit-masked with the
 origin and destination shard identifiers respectively. [qip-0002](qip-0002.md)
 describes the encoding of shard identifiers. For this specification, we only
 care that they are 9 bits in length.
 
-| bits 0..8    | bits 9..17   | bits 18..255               |
-|--------------|--------------|----------------------------|
-| src shard id | dst shard id | hash(transaction)[18..255] |
+| bits 0..8    | bits 9..15               | bits 16..24  | bits 25..255               |
+|--------------|--------------------------|--------------|----------------------------|
+| src shard id | hash(transaction)[9..15] | dst shard id | hash(transaction)[25..255] |
+
+> Note: since shard identifiers are 9 bits, we include a 7 bits of the actual
+> transaction hash as a spacer, so that the destination shard id starts at a
+> round byte position.
 
 ## Copyright
 This QIP licensed under the BSD 2-clause license.