Update while-let-expressions.md (#1018)

Update code snippet description to use `v.into_iter()`
instead of `v.iter()` since the former is what's actually
used in the code snippet.

src/control-flow/while-let-expressions.md

@@ -14,7 +14,7 @@ fn main() {
 }
 ```
 
-Here the iterator returned by `v.iter()` will return a `Option<i32>` on every
+Here the iterator returned by `v.into_iter()` will return a `Option<i32>` on every
 call to `next()`. It returns `Some(x)` until it is done, after which it will
 return `None`. The `while let` lets us keep iterating through all items.