Fix syntax in Expr.scala
Fix syntax in Expr.scala

```
class Expr private

object
```
now means the `private` modifier is for the following `object`, not the (invisible) constructor of `Expr`.

dotty-staging/dotty,sjrd/dotty,lampepfl/dotty,dotty-staging/dotty,lampepfl/dotty,sjrd/dotty,sjrd/dotty,lampepfl/dotty,dotty-staging/dotty,lampepfl/dotty,dotty-staging/dotty,sjrd/dotty,sjrd/dotty,dotty-staging/dotty,lampepfl/dotty