Wrong example about the https://github.com/Azure/openapi-diff/blob/master/docs/rules/1029.md

Hi team,

In the doc https://github.com/Azure/openapi-diff/blob/master/docs/rules/1029.md,
the Example is:
```
Example: readonly of property message **removed** in the new version.
```

However compared with the old specification, the new specification **added** "readonly" property

開啟 docs/rules/1029.md，並比較範例與規則所描述的舊版和新版 OpenAPI 規格之間的變更。修正範例，使其準確說明 readonly 屬性是被新增還是被移除，然後確認轉譯後的 Markdown 文字是否清楚。

由索引模型根據 Issue 內容生成。