Documentation error: typo in "docs/auth/getting-started.md"

### How to reproduce these conditions

- Open "[docs/auth/getting-started.md](https://github.com/angular/angularfire/blob/master/docs/auth/getting-started.md)"
- Search for "Read more at authentication state persistence"

### Expected behavior
The sentence ends with a period.

### Actual behavior
The sentence ends with a comma.

開啟 docs/auth/getting-started.md 並搜尋「Read more at authentication state persistence」。檢查該句結尾的標點，並更新它，使文件以預期的標點結尾。當該句符合 issue 預期的行為時，即表示完成。

由索引模型根據 Issue 內容生成。