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.

打开 docs/auth/getting-started.md 并搜索“Read more at authentication state persistence”。检查该句末尾的标点，并更新它，使文档以预期的标点结尾。当该句与 issue 预期的行为一致时，即表示完成。

由索引模型根据 Issue 内容生成。