message: update README.md
  The readme `migrate` signature is wrong at it does not accept a `MessageInfo`.
diff: @@ -159,7 +159,7 @@ functionality:
  ```rust
  // in-place contract migrations
  -extern "C" fn migrate(env_ptr: u32, info_ptr: u32, msg_ptr: u32) -> u32;
  +extern "C" fn migrate(env_ptr: u32, msg_ptr: u32) -> u32;
  // support submessage callbacks
  extern "C" fn reply(env_ptr: u32, msg_ptr: u32) -> u32;
  