Feedback for “Lesson 3 - Basic anatomy of a protocol in Quint”

The following line:

```code
pure val MAX_UINT = 2^256 - 1
```
doesn't work from the quint REPL (Ubuntu Linux, 64-bit).

To get going, I gone ahead with this:

```pure val MAX_UINT = 9223372036854775807```

If this is expected, then the lesson should me it clear. 

Just a suggestion.