Example for expand/collapse is broken

Hello,

In the docs there is an example for expand/collapse.
I justed wanted to let you know that the actionButton "expandWAfrica" cannot work, because ```nodes_input = TRUE``` isn't set in ``` treeviewInput```.

Further I do not know if it is the intented behaviour that then the countrys of level 3 are then displayed directly under level 1. I would assume level 2 (e.g. western africa) is still displayed (but now expanded).

Start with the documentation's expand/collapse example and inspect the treeviewInput setup around the expandWAfrica actionButton. Verify that the example works, then determine and document whether level 2 remains visible when level 3 countries are expanded.

Written by the indexing model from the issue text.