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.

Open docs/auth/getting-started.md and search for "Read more at authentication state persistence". Check the punctuation at the end of that sentence and update it so the documentation ends with the expected punctuation. Done means the sentence matches the issue's expected behavior.

Written by the indexing model from the issue text.