This was indented two spaces too much and the line should be broken up into at least two lines. Also, the url should get moved out of the translated text.

So something like this:

```
{% trans contributors=user_list(contributors) url='https://support.mozilla.org/get-involved' %}
  These fine people helped write this article:: {{ contributors }}. You can help too - find out
  <a href="{{ url }}">how</a>. 
{% endtrans %}
```