fix(example): correct markup in the hello world example

Earlier version had multiline template and backticks were needed (as Pavel pointed out). I am ok with changing to `'` if we think that is less confusing.

@pkozlowski-opensource `${greeting}` would just fail since the annotation is evaluated in the global scope and there is no `greeting` symbol there. It is not evaluated on the instance, like angular expressions. That said the following would work just fine:

```
const TAG = 'span'

@Component({
  ...
  inline: `{{greeting}} <${TAG} red>world</${TAG}>!`,
```

But I agree it is confusing to have two types of expressions in the same template, so I would avoid it unless necessary.