Update treemaps.md

Author: LiamConnors

doc/python/treemaps.md

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     description: How to make Treemap Charts with Plotly
     display_as: basic
@@ -145,6 +145,23 @@ fig.update_layout(margin = dict(t=50, l=25, r=25, b=25))
 fig.show()
 ```
 
+### Treemap with Rounded Corners
+
+
+*New in 5.12*
+
+Update treemap sectors to have rounded corners by configuring the `cornerradius` in px.
+
+```python
+import plotly.express as px
+fig = px.treemap(
+    names = ["Eve","Cain", "Seth", "Enos", "Noam", "Abel", "Awan", "Enoch", "Azura"],
+    parents = ["", "Eve", "Eve", "Seth", "Seth", "Eve", "Eve", "Awan", "Eve"]
+)
+fig.update_traces(marker={"cornerradius": 5})
+fig.show()
+```
+
 ### Basic Treemap with go.Treemap
 
 If Plotly Express does not provide a good starting point, it is also possible to use [the more generic `go.Treemap` class from `plotly.graph_objects`](/python/graph-objects/).