fix comment on message memory layout

contracts/bridge.sol

@@ -123,11 +123,11 @@ library Message {
     // offset 84: 32 bytes :: bytes32 - transaction hash
     // offset 116: 32 bytes :: uint256 (big endian) - home gas price
 
-    // bytes 1 to 32 are 0 because message length is stored as little endian.
     // mload always reads 32 bytes.
+    // if mload reads an address it only interprets the last 20 bytes as the address.
     // so we can and have to start reading recipient at offset 20 instead of 32.
     // if we were to read at 32 the address would contain part of value and be corrupted.
-    // when reading from offset 20 mload will read 12 zero bytes followed
+    // when reading from offset 20 mload will ignore 12 bytes followed
     // by the 20 recipient address bytes and correctly convert it into an address.
     // this saves some storage/gas over the alternative solution
     // which is padding address to 32 bytes and reading recipient at offset 32.