Fix missing code block for some scala code.

week1/01-example.md

@@ -198,11 +198,13 @@ res1: Int = 3
 You can enter a multiline expression in the REPL by using `Alt+Enter`
 (`Option+Enter` on macOS) instead of `Enter`:
 
+```scala
 scala> if 1 == 1 then
      |   "a"
      | else
      |   "b"
 val res0: String = a
+```
 
 In order to exit the Scala REPL and go back to sbt, type `Ctrl+D`.