Fix reference links

Classes/Smart casts/task.md

@@ -1,7 +1,7 @@
 ## Smart casts
 
 Rewrite the following Java code using [smart casts](https://kotlinlang.org/docs/typecasts.html#smart-casts)
-and the [when](https://kotlinlang.org/docs/control-flow.html#when-expression) expression:
+and the [when](https://kotlinlang.org/docs/control-flow.html#when-expressions-and-statements) expression:
 
 ```java
 public int eval(Expr expr) {

Introduction/Default arguments/task.md

@@ -19,4 +19,4 @@ public String foo(String name) {
 
 You can replace all these Java overloads with one function in Kotlin.
 Change the declaration of the `foo` function in a way that makes the code using `foo` compile.
-Use [default and named](https://kotlinlang.org/docs/functions.html#default-arguments) arguments.
+Use [Parameters with default values](https://kotlinlang.org/docs/functions.html#parameters-with-default-values).

Introduction/Named arguments/task.md

@@ -3,8 +3,8 @@
 Make the function `joinOptions()` return the list in a JSON format (for example, `[a, b, c]`)
 by specifying only two arguments. 
 
-[Default and named](https://kotlinlang.org/docs/functions.html#default-arguments)
-arguments help to minimize the number of overloads and improve the readability of the function invocation.
+[Named arguments](https://kotlinlang.org/docs/functions.html#named-arguments) and [Parameters with default values](https://kotlinlang.org/docs/functions.html#parameters-with-default-values)
+ help to minimize the number of overloads and improve the readability of the function invocation.
 The library function [`joinToString`](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.collections/join-to-string.html)
 is declared with default values for parameters: