Incorrect documentation example for unnecessary_filter_map

[lily-commure]

The documentation for unnecessary_filter_map gives

let _ = (0..4).filter_map(i32::checked_abs);

as an example of a filter_map that can be replaced by a map. However, it is incorrect.

As can be seen from the test and its associated output, that line is actually an example of good code which should not be caught by the lint.

Probably let _ = (0..4).filter_map(|x| Some(x + 1)); would be a good example to use in the documentation instead, as it can be replaced by let _ = (0..4).map(|x| x + 1);