configure script: packages with dashes (-) will create invalid PHP code in README usage section

[levu42]

Hello! If I create a package with a - in the package name, the usage section in the README uses the dashes directly for a variable name, resulting in incorrect PHP.

Example: Running the configure script with vendor = Me, package name = my-first-package, class name = MFP results in:

## Usage
```php
$my-first-package = new Me\MPF();

when it should convert the variable name e.g. with slugify to $myfirstpackage.

Thanks for the great video course & this helpful skeleton!

[freekmurze]

Feel free to submit a PR that fixes this.

Thanks for the kind words on our work 👍