Correct using directives on configuration.md (#1278)

I came across the `//> using`-style directive in this page and it didn't work. The syntax on https://scala-cli.virtuslab.org/docs/guides/using-directives/ seems to work so I updated them here.

Author: megri

website/docs/guides/configuration.md

@@ -46,8 +46,8 @@ This is achieved by specifying `using` directives inside comments at the top of
 
 ```scala
 //> using scala "2.13"
-//> using scala-js
-//> using options -Xasync
+//> using platform "scala-js"
+//> using options "-Xasync"
 
 // package and import statements follow here ...
 ```
@@ -67,4 +67,4 @@ import ujson._
 
 Both `import $ivy` and `import $dep` are accepted, and are equivalent. Note that this syntax
 might be deprecated - and then removed - in the future. It's recommended to add dependencies
-with [`using` directives](#using-directives).
+with [`using` directives](#using-directives).