inverted fields in `hex::Error::InvalidLength`

I am testing https://github.com/ElementsProject/rust-elements/pull/188 in a downstream lib

https://github.com/ElementsProject/rust-elements/blob/613204f38eeb8f4fcc767042197fd763965f057a/src/hex.rs#L33

I think the `expected, got` comment is wrong and the values used in the Display implementation are inverted.

I say so by looking at the macro impl after in a downstream lib I have this test failing to parse "aaaa" as asset id with

```
left: "bad hex string length 64 (expected 4)"
right: "bad hex string length 4 (expected 64)"
```

(the test was working with add54c80ba1821a16b88af59de16009f28379853)

從 src/hex.rs 約第 33 行開始，檢查 InvalidLength 註解、Display 實作和巨集實作。重現 downstream asset-id 對 "aaaa" 的解析情境，並比較預期的錯誤措辭；當實際長度和預期長度依照預定順序回報且註解一致時，即表示完成。

由索引模型根據 Issue 內容生成。