Fix issue #1980: opened_symbol option in TreeLayerControl not functioning (#1981)

Co-authored-by: James Doyle <James.Doyle@resonate.tech>

Author: Doyley1

folium/plugins/treelayercontrol.py

@@ -151,7 +151,7 @@ def __init__(
         super().__init__()
         self._name = "TreeLayerControl"
         kwargs["closed_symbol"] = closed_symbol
-        kwargs["openened_symbol"] = opened_symbol
+        kwargs["opened_symbol"] = opened_symbol
         kwargs["space_symbol"] = space_symbol
         kwargs["selector_back"] = selector_back
         kwargs["named_toggle"] = named_toggle