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`.

library/src/scala/quoted/Expr.scala

@@ -4,7 +4,7 @@ package scala.quoted
  *
  *  `Expr` has extension methods that are defined in `scala.quoted.Quotes`.
  */
-abstract class Expr[+T] private[scala]
+abstract class Expr[+T] private[scala] ()
 
 /** Constructors for expressions */
 object Expr {