docs(ch13-02): fix typo 'iteration' to 'iterator' in filter example

Fixes #4705

The line "the value will be included in the iteration produced by
filter" should refer to "the iterator produced by filter", matching
the surrounding paragraph's use of "iterator" and matching what
filter actually returns (an iterator that lazily yields the chosen
items).

Verified against the current rendering on the stable book page:
https://doc.rust-lang.org/stable/book/ch13-02-iterators.html#closures-that-capture-their-environment

Only one word is changed. `dprint fmt` reports no further formatting
changes.

src/ch13-02-iterators.md

@@ -196,7 +196,7 @@ their environment.
 
 For this example, we’ll use the `filter` method that takes a closure. The
 closure gets an item from the iterator and returns a `bool`. If the closure
-returns `true`, the value will be included in the iteration produced by
+returns `true`, the value will be included in the iterator produced by
 `filter`. If the closure returns `false`, the value won’t be included.
 
 In Listing 13-16, we use `filter` with a closure that captures the `shoe_size`