fix trimming of result lines

Result lines now trimmed by 3 characters ("<p>") in the beginning instead of 5, but the "=>" is removed as well.
If the result line is empty, no `<div>` is created, so

```css
.apiref-result:before {
  content: "=> ";
}
```
can be used to restore the "=>" or add a different marker to the line.