import Alert component for example code block

[orcuntuna]

PR Checklist

• The PR title follows our guidelines: https://github.com/proyecto26/react-native-inappbrowser/blob/master/CONTRIBUTING.md#pull-request-process.
• There is an issue for the bug/feature this PR is for. To avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it.
• All existing tests are passing
• Tests for the changes are included

What is the current behavior?

The alert component was not imported in the sample code snippet on readme file.

What is the new behavior?

I imported the Alert component

Fixes/Implements/Closes #[Issue Number].