Fix package name

I tried including this earlier, but got errors like:

```
Error: Execution of '/usr/sbin/chown -R technicalpickles:staff /Applications/Slack.app.app' returned 1: chown: /Applications/Slack.app.app: No such file or directory

Error: /Stage[main]/Slack/Package[Slack.app]/ensure: change from absent to present failed: Execution of '/usr/sbin/chown -R technicalpickles:staff /Applications/Slack.app.app' returned 1: chown: /Applications/Slack.app.app: No such file or directory
```

Looking at the manifest, it was specifying `Slack.app`, which the package provider 'updated' to 'Slack.app.app', which wouldn't exist. This updates the package name to be just `Slack` which appears to work as expected.