[reference] Update 7.2.20: For expressions.

* `for` loops now use `IntoIterator` instead of just `Iterator`
* Simplify the example by removing unnecessary `Vec::iter` call.

src/doc/reference.md

@@ -3192,7 +3192,7 @@ for_expr : [ lifetime ':' ] "for" pat "in" no_struct_literal_expr '{' block '}'
 ```
 
 A `for` expression is a syntactic construct for looping over elements provided
-by an implementation of `std::iter::Iterator`.
+by an implementation of `std::iter::IntoIterator`.
 
 An example of a for loop over the contents of an array:
 
@@ -3205,8 +3205,8 @@ An example of a for loop over the contents of an array:
 
 let v: &[Foo] = &[a, b, c];
 
-for e in v.iter() {
-    bar(*e);
+for e in v {
+    bar(e);
 }
 ```