Improve NotGiven example

- Use new Scala 3 syntax uniformly
- Don't use a low priority trait since it's a complication not needed
  for the example

Author: smarter

docs/docs/reference/contextual/givens.md

@@ -110,30 +110,23 @@ In each case, a pattern-bound given instance consists of `given` and a type `T`.
 
 Scala 2's somewhat puzzling behavior with respect to ambiguity has been exploited to implement the analogue of a "negated" search in implicit resolution, where a query Q1 fails if some other query Q2 succeeds and Q1 succeeds if Q2 fails. With the new cleaned up behavior these techniques no longer work. But there is now a new special type `scala.util.NotGiven` which implements negation directly.
 
-For any query type Q: NotGiven[Q] succeeds if and only if the implicit search for Q fails. Example:
+For any query type `Q`, `NotGiven[Q]` succeeds if and only if the implicit
+search for `Q` fails, for example:
 
 ```scala
-
 import scala.util.NotGiven
 
 trait Tagged[A]
 
 case class Foo[A](value: Boolean)
-trait FooLowPrio {
-  implicit def fooDefault[A]: Foo[A] = Foo(true)
-}
-
-object Foo extends FooLowPrio {
-  implicit def fooNotTagged[A](implicit ev: NotGiven[Tagged[A]]): Foo[A] = Foo(false)
-}
-
-@main def main() =
-  given Tagged[Int] = null
-
-  assert(implicitly[Foo[Int]].value) // fooDefault
-
-  assert(!implicitly[Foo[String]].value) // fooNotTagged
-
+object Foo:
+  given fooTagged[A](using Tagged[A]): Foo[A] = Foo(true)
+  given fooNotTagged[A](using NotGiven[Tagged[A]]): Foo[A] = Foo(false)
+
+@main def test() =
+  given Tagged[Int] with {}
+  assert(implicitly[Foo[Int]].value) // fooTagged is found
+  assert(!implicitly[Foo[String]].value) // fooNotTagged is found
 ```
 
 ## Given Instance Initialization