Fix BTreeMap example typo
The whole example is made around movies reviews, but that one line says "review some books".
/// // would be `BTreeMap<&str, &str>` in this example).
/// let mut movie_reviews = BTreeMap::new();
///
-/// // review some books.
+/// // review some movies.
/// movie_reviews.insert("Office Space", "Deals with real issues in the workplace.");
/// movie_reviews.insert("Pulp Fiction", "Masterpiece.");
/// movie_reviews.insert("The Godfather", "Very enjoyable.");