[DOC] Misleading doc in SlidingWindowForecastCV
As mentioned in the comment section of #201, there is a misleading statement in the [`SlidingWindowForecastCV`](https://github.com/tgsmith61591/pmdarima/blob/master/pmdarima/model_selection/_split.py#L200):

```
    step : int, optional (default=1)
        The size of step taken to increase the training sample size when not
        performing using a window.
```

This is an artifact of when both CV strategies were lumped into the same class. This should read:

```
    step : int, optional (default=1)
        The size of step taken between training folds.
```

Good issue for anyone looking to get involved.
diff --git a/pmdarima/model_selection/_split.py b/pmdarima/model_selection/_split.py
index cb2b3430..6a0f46ce 100644
--- a/pmdarima/model_selection/_split.py
+++ b/pmdarima/model_selection/_split.py
@@ -198,8 +198,7 @@ class SlidingWindowForecastCV(BaseTSCrossValidator):
         the last training sample for the test set.
 
     step : int, optional (default=1)
-        The size of step taken to increase the training sample size when not
-        performing using a window.
+        The size of step taken between training folds.
 
     window_size : int or None, optional (default=None)
         The size of the rolling window to use. If None, a rolling window of
['pmdarima/model_selection/_split.py']