Fix for incorrect launchable tag in metainfo file

[dex4er]

FIxes incorrect <launchable> tag as it was discovered in https://buildbot.flathub.org/#/builders/6/builds/177149:

{
    "errors": [
        "metainfo-launchable-tag-wrong-value"
    ],
    "info": [
        "metainfo-launchable-tag-wrong-value: The value of launchable tag in Metainfo is wrong: app.freelens.Freelens"
    ],
    "message": "Please consult the documentation at https://docs.flathub.org/docs/for-app-authors/linter"
}

[dex4er]

I'm not sure if it is so important that we should release a new package soon.