Rephrase definition of single-expression function

The current definition says "When a function returns a single expression", which I don't think is correct: It sounds as if the function returns the expresions inside it.

This is an opinionated nitpick, though, so feel free to close this!

docs/topics/functions.md

@@ -177,7 +177,7 @@ fun printHello(name: String?) { ... }
 
 ### Single-expression functions
 
-When a function returns a single expression, the curly braces can be omitted and the body is specified after a `=` symbol:
+When a function contains a single `return` expression, the curly braces can be omitted and the body is specified after a `=` symbol:
 
 ```kotlin
 fun double(x: Int): Int = x * 2