Update to latest manifest.json

[coderDojoFaith]

https://github.com/sahilsehwag/pluralsight-downloader-chrome-extension

Error: manifest version 3 is latest, not able to read manifest version 2

'background.persistent' requires manifest version of 2 or lower.
'background.scripts' requires manifest version of 2 or lower.
'browser_action' requires manifest version of 2 or lower.