[Minor Fix][Doc]What's New in 1.0.0
#### Problem description
Minor typo under the [Dedicated-string-data-type](https://dev.pandas.io/docs/whatsnew/v1.0.0.html#dedicated-string-data-type) warning, it should be `StringDtype is currently considered experimental.` instead of `StringDtype` ~and~ `is currently considered experimental.`