Set jitsi server in element desktop conf not working

I have Element 1.7.29 on Pop OS 20.10
I did
```
nano ~/.config/Element/config.json
```
The file was not existing. I copy paste in it the following, as explained in here https://github.com/vector-im/element-web/blob/develop/docs/jitsi.md
```
{
"jitsi": {
"preferredDomain": "jitsi.xxx.fr"
}
}
```

then I restarted Element and when I click on camera call button in a room, it creates a jitsi.riot.im call and not a jitsi.xxx.fr call as expected.