Meteor 3 writable shrinkwrap.json issue is fixed

This is a reminder that the issue mentioned in the README.md file should no longer occur.

```
- Mostly supports Meteor 3 (requires making programs/server/shrinkwrap.json writable due to a bug in Meteor 3)
```

The issue was resolved some time ago as part of Meteor 3 core updates: https://github.com/meteor/meteor/pull/13219.