Proposal: rename `$appDir` to `$rootDir` in Bootstrap.php

The `$appDir` variable in the `app/Bootstrap.php` corresponds to the `$rootDir` variable in `Container` and not corresponds to `$appDir` in `Container`.
This inconsistency can be confusing.

[Default parameters in Container](https://doc.nette.org/en/application/bootstrap#toc-default-parameters)