Add example of using the indexing operator to HashMap docs

Author: fhartwig

src/libstd/collections/hash/map.rs

@@ -318,6 +318,9 @@ const DISPLACEMENT_THRESHOLD: usize = 128;
 ///     }
 /// }
 ///
+/// // Look up the value for a key (will panic if the key is not found).
+/// println!("Review for Jane: {}", book_reviews["Pride and Prejudice"]);
+///
 /// // Iterate over everything.
 /// for (book, review) in &book_reviews {
 ///     println!("{}: \"{}\"", book, review);