update char_indices example to highlight big chars

src/liballoc/str.rs

@@ -719,13 +719,17 @@ impl str {
     /// Remember, [`char`]s may not match your human intuition about characters:
     ///
     /// ```
-    /// let y = "y̆";
+    /// let yes = "y̆es";
     ///
-    /// let mut char_indices = y.char_indices();
+    /// let mut char_indices = yes.char_indices();
     ///
     /// assert_eq!(Some((0, 'y')), char_indices.next()); // not (0, 'y̆')
     /// assert_eq!(Some((1, '\u{0306}')), char_indices.next());
     ///
+    /// // note the 3 here - the last character took up two bytes
+    /// assert_eq!(Some((3, 'e')), char_indices.next());
+    /// assert_eq!(Some((4, 's')), char_indices.next());
+    ///
     /// assert_eq!(None, char_indices.next());
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]