Update kotlin.md (#2096)

Added return type on method.

Co-authored-by: Rico Sta. Cruz <rstacruz@users.noreply.github.com>

kotlin.md

@@ -224,7 +224,7 @@ val four = 2.timesTwo()
 ### Default Parameters
 
 ```kotlin
-fun getGreeting(person: Person, intro: String = "Hello,") {
+fun getGreeting(person: Person, intro: String = "Hello,"): String {
     return "$intro ${person.name}"
 }