Update live demo to 8.0.0 syntax (`new Level`)

[SebastianSimon]

The breaking change of version 8.0.0 requires a switch from

const level = require('level')
const db = level('db')

to

const { Level } = require('level')
const db = new Level('db')

The demo appears to still refer to 5.0.0. The outdatedness sparks Stack Overflow questions such as Error: "level is not a function" when using LevelDB.

[vweevers]

Fixed, thanks for bringing that up!

[SebastianSimon]

That was quick, thank you! I’ve noticed that the text below the sample still says “As of level@5, the above code works in Node.js, Electron and browsers!”, though. That should be level@8 now, right?

[vweevers]

That's technically still true, although less important to mention now. We'll remove it.

Generally need some cleanup here. I've opened #64.