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