Readme install instructions out of date, no longer need node --experimental-wasm-threads --experimental-wasm-bulk-memory transcode.js as of Node 12.x

The readme install is outdated, running the node experimental commands give bad option.

These are default in Node 12.x + so get rid of that line (below) or add a disclaimer it's node versions less than 12.

```
As we are using the latest experimental features, you need to add few flags to run in Node.js

$ node --experimental-wasm-threads --experimental-wasm-bulk-memory transcode.js
```

Successfully installed ffmpeg.wasm with only the npm install command with node 16.