x/website: changes in code not highlighted in grettings-multiple-people tutorial

### What did you expect to see?

On the site https://golang.org/doc/tutorial/greetings-multiple-people Step 2  in the code block:

```
// Request greeting messages for the names.
    messages, err := greetings.Hellos(names)
```

I think the font color should be blue, because this part changed from "Hello" to  "Hellos".

### What did you see instead?

Font color is black. 
