Fix code in 'Scala Tour / Singleton Objects / Companion Objects'

Author: VMironiuk

_tour/singleton-objects.md

@@ -104,6 +104,7 @@ An object with the same name as a class is called a _companion object_. Converse
 
 {% tab 'Scala 2' for=companion-object-circle %}
 ```scala
+import scala.math.Pi
 import scala.math.pow
 
 case class Circle(radius: Double) {
@@ -123,7 +124,7 @@ circle1.area
 
 {% tab 'Scala 3' for=companion-object-circle %}
 ```scala
-import scala.math.pow
+import scala.math.{Pi, pow}
 
 case class Circle(radius: Double):
   import Circle.*