The list in ml-ensembles.md wasn't properly formatted and, as a result, was looking like this:
![old](http://i.imgur.com/2ZhELLR.png)

This PR aims to make it look like this:
![new](http://i.imgur.com/0Xriwd2.png)

Author: BenFradet <benjamin.fradet@gmail.com>

Closes #10025 from BenFradet/ml-ensembles-doc.