Add missing import in example code

Author: mozz100

packages/plotly-extension/README.md

@@ -48,6 +48,7 @@ Plotly(data, layout)
 To render using the [plotly Python library](https://github.com/plotly/plotly.py):
 
 ```python
+import plotly
 from plotly.offline import iplot
 
 trace = plotly.graph_objs.Heatmap(z=[[1, 20, 30],