Update manifest.json forgot comma

[raszpl]

I was going to also fix FF compatibility, but:
with "service_worker": changed to "scripts": https://stackoverflow.com/questions/75043889/manifest-v3-background-scripts-service-worker-on-firefox/78088358#78088358 needs both

"background": {
    "service_worker": "background.js",
    "scripts": ["background.js"]
},

for full cross compatibility with Firefox under V3 manifest
https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/background#browser_support
Issue: warnins in both browsers :/. Firefox one:

Warning details

Reading manifest: Warning processing background.service_worker: An unexpected property was found in the WebExtension manifest.

Chrome one:
'background.scripts' requires manifest version of 2 or lower.

[ImprovedTube]

just to confuse: Firefox can stay manifest2 it seems.