Modified the example to demonstrate 'sumBy'
Modified the example to demonstrate 'sumBy'

Riccorl/kotlin-koans,JetBrains/workshop-jb,zyallday/kotlin-koans,trmollet/tuto_kotlin,enchf/kotlin-koans,Carighan/kotlin-koans,Kotlin/kotlin-koans,Kotlin/kotlin-koans,enchf/kotlin-koans,trmollet/tuto_kotlin,TheCoinTosser/Kotlin-Koans---Solutions,JetBrains/workshop-jb,hughandrewarch/kotlin-koans,TheCoinTosser/Kotlin-Koans---Solutions,Riccorl/kotlin-koans,hughandrewarch/kotlin-koans,zyallday/kotlin-koans,Carighan/kotlin-koans