chore: update implicit this expression sample (#4552)

Author: koshachy

docs/topics/this-expressions.md

@@ -45,8 +45,7 @@ If you have a non-member function with the same name, use this with caution beca
 
 ```kotlin
 fun main() {
-//sampleStart
-    fun printLine() { println("Top-level function") }
+    fun printLine() { println("Local function") }
 
     class A {
         fun printLine() { println("Member function") }
@@ -58,8 +57,7 @@ fun main() {
     }
 
     A().invokePrintLine() // Member function
-    A().invokePrintLine(omitThis = true) // Top-level function
-//sampleEnd()
+    A().invokePrintLine(omitThis = true) // Local function
 }
 ```
 {kotlin-runnable="true" kotlin-min-compiler-version="1.3"}