update docs

Author: bishabosha

docs/docs/reference/enums/desugarEnums.md

@@ -175,7 +175,8 @@ Companion objects of enumerations that contain at least one simple case define i
      ```scala
      private def $new(_$ordinal: Int, $name: String) = new E {
        def $ordinal = $_ordinal
-       override def toString = $name
+       override def productPrefix = $name
+       override def toString = productPrefix
        $values.register(this) // register enum value so that `valueOf` and `values` can return it.
      }
      ```

docs/docs/reference/enums/enums.md

@@ -130,7 +130,8 @@ For instance, the `Venus` value above would be defined like this:
 val Venus: Planet =
   new Planet(4.869E24, 6051800.0) {
     def ordinal: Int = 1
-    override def toString: String = "Venus"
+    override def toString: String = productPrefix
+    override def productPrefix: String = "Venus"
     // internal code to register value
   }
 ```