core: Fix example for .map()
core: Fix example for .map()

Make the example use DoubleEndedIterator for map, like it said it would.

graydon/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,graydon/rust,GBGamer/rust,graydon/rust,GBGamer/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,GBGamer/rust,aidancully/rust,aidancully/rust