Fix promotion of top-level checkbox not removing it properly

[mario-holubar]

The issue this tries to solve:

- [ ] abc

Calling promote here previously only removed the dash, not the brackets.

Explanation:

parse_bullet matches and returns both the bullet (with text_after_bullet being [ ] abc) and the checkbox (with text_after_bullet being abc).
When promoting, change_line_bullet_level was considering the first item returned by parse_bullet and thus keeping the text after the bullet, including the [ ].
I changed it to use the last returned item so that only the text after the checkbox is kept.

[mario-holubar]

I haven't verified that this doesn't break something else btw as I haven't been able to set up the test environment on my machine