Update CONTRIBUTING.md to reflect enum Step sort enforcement (#1111)

* Update CONTRIBUTING.md

* Formatting

CONTRIBUTING.md

@@ -20,11 +20,11 @@ To add a new `step` to `topgrade`:
 
    ```rust
    pub enum Step {
-       // Existed steps
+       // Existing steps
        // ...
 
        // Your new step here!
-       // You may want it to be sorted alphabetically because that looks great:)
+       // Make sure it stays sorted alphabetically because that looks great :)
        Xxx,
    }
    ```
@@ -78,7 +78,7 @@ To add a new `step` to `topgrade`:
    to separate the steps, for example, for steps that are Linux-only, it goes
    like this:
 
-   ```
+   ```rust
    #[cfg(target_os = "linux")]
    {
        // Xxx is Linux-only