Fixes popTo signature (#1973)

The only argument to `popTo` is the containerId to pop to, not the current containerId as well.  Also showing `this.props.previousContainerIds` lead me to believe that was included automatically (similar to `this.props.containerId`) so I changed that to avoid confusion.

docs/docs/usage.md

 ```js
 Navigation.pop(this.props.containerId);
 ```
-### popTo(params)
+### popTo(containerId)
 ```js
-Navigation.popTo(this.props.containerId, this.props.previousScreenIds[0]);
+Navigation.popTo(previousScreenId);
 ```
 ### popToRoot()
 Pop all the screens until the root from this screen's navigation stack