kotlin-tour-functions.md : fix filter predicate explanation (#4925)

* kotlin-tour-functions.md : fix filter predicate explanation

The predicate doesn't do the filtering, .filter() does

* Apply suggestions from code review

Co-authored-by: Sarah Haggarty <[email protected]>

---------

Co-authored-by: Sarah Haggarty <[email protected]>

Author: julienbraure

docs/topics/tour/kotlin-tour-functions.md

@@ -396,10 +396,10 @@ fun main() {
 ```
 {kotlin-runnable="true" kotlin-min-compiler-version="1.3" id="kotlin-tour-lambda-filter"}
 
-The `.filter()` function accepts a lambda expression as a predicate:
+The `.filter()` function accepts a lambda expression as a predicate and applies it to each element of the list. The function keeps an element only if the predicate returns `true`:
 
-* `{ x -> x > 0 }` takes each element of the list and returns only those that are positive.
-* `{ x -> x < 0 }` takes each element of the list and returns only those that are negative.
+* `{ x -> x > 0 }` returns `true` if the element is positive.
+* `{ x -> x < 0 }` returns `true` if the element is negative.
 
 This example demonstrates two ways of passing a lambda expression to a function: