[MINOR][DOCS] fixed list display in ml-ensembles

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 <[email protected]>

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

Author: BenFradet

docs/ml-ensembles.md

@@ -20,6 +20,7 @@ Both use [MLlib decision trees](ml-decision-tree.html) as their base models.
 Users can find more information about ensemble algorithms in the [MLlib Ensemble guide](mllib-ensembles.html).  In this section, we demonstrate the Pipelines API for ensembles.
 
 The main differences between this API and the [original MLlib ensembles API](mllib-ensembles.html) are:
+
 * support for ML Pipelines
 * separation of classification vs. regression
 * use of DataFrame metadata to distinguish continuous and categorical features