Minor fixes (#579)

* order the Cat and Dog correctly in the vec

* make example editable

* patterns: capture vs. constant

* clarify notes on indexing a string

src/std/string.md

@@ -35,6 +35,8 @@ fn main() {
     * `String` implements `Deref<Target = str>` which transparently gives it access to `str`'s methods.
     * Write and compare `let s3 = s1.deref();` and  `let s3 = &*s1`;.
 * `String` is implemented as a wrapper around a vector of bytes, many of the operations you see supported on vectors are also supported on `String`, but with some extra guarantees.
-* Compare the different ways to index a `String` by using `s3[i]` and `s3.chars().nth(i).unwrap()` where `i` is in-bound, out-of-bounds, and "on" the flag Unicode character.
+* Compare the different ways to index a `String`:
+    * To a character by using `s3.chars().nth(i).unwrap()` where `i` is in-bound, out-of-bounds.
+    * To a substring by using `s3[0..4]`, where that slice is on character boundaries or not.
 
 </details>