doc: update docs/kotlin.md (#482)

* doc: update

* fix: kotlin 方法默认参数指定其类型；

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Co-authored-by: 小弟调调™ <kennyiseeyou@gmail.com>

docs/kotlin.md

@@ -523,7 +523,7 @@ fun main() {
 ### 默认参数
 
 ```kotlin
-fun favoriteLanguage(name, language = "Kotlin") {
+fun favoriteLanguage(name: String, language: String = "Kotlin") {
   println("Hello, $name. Your favorite programming language is $language")  
 }