[MINOR][ML] Rename TreeEnsembleModels to TreeEnsembleModel for PySpark

## What changes were proposed in this pull request?
Fix the typo of ```TreeEnsembleModels``` for PySpark, it should ```TreeEnsembleModel``` which will be consistent with Scala. What's more, it represents a tree ensemble model, so  ```TreeEnsembleModel``` should be more reasonable. This should not be used public, so it will not involve  breaking change.

## How was this patch tested?
No new tests, should pass existing ones.

Author: Yanbo Liang <ybliang8@gmail.com>

Closes #14454 from yanboliang/TreeEnsembleModel.