Correct terminology from "first-order functions" to "first-class functions"

Author: yyh-gl

docs/src/md/kotlin.core/type-system.md

@@ -794,7 +794,7 @@ In case we need to establish type containment between regular type $A$ and captu
 
 #### Function types
 
-Kotlin has first-order functions; e.g., it supports function types, which describe the argument and return types of its corresponding function.
+Kotlin has first-class functions; e.g., it supports function types, which describe the argument and return types of its corresponding function.
 
 A function type $\FT$