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/pattern-matching/destructuring-structs.md

@@ -9,4 +9,8 @@ You can also destructure `structs`:
 
 * Change the literal values in `foo` to match with the other patterns.
 * Add a new field to `Foo` and make changes to the pattern as needed.
+* The distinction between a capture and a constant expression can be hard to
+  spot. Try changing the `2` in the second arm to a variable, and see that it subtly
+  doesn't work. Change it to a `const` and see it working again.
+
 </details>

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>

src/traits/trait-objects.md

@@ -2,7 +2,7 @@
 
 Trait objects allow for values of different types, for instance in a collection:
 
-```rust
+```rust,editable
 trait Pet {
     fn name(&self) -> String;
 }
@@ -27,8 +27,8 @@ impl Pet for Cat {
 
 fn main() {
     let pets: Vec<Box<dyn Pet>> = vec![
-        Box::new(Dog { name: String::from("Fido") }),
         Box::new(Cat),
+        Box::new(Dog { name: String::from("Fido") }),
     ];
     for pet in pets {
         println!("Hello {}!", pet.name());
@@ -80,4 +80,4 @@ Memory layout after allocating `pets`:
          println!("{}", std::mem::size_of::<Box<dyn Pet>>());
      ```
 
-</details>
+</details>