Add ipywidgets install for plotly

I am using just an install of JupyterLab, not through Anaconda, and I needed to also install `ipywidgets` for Plotly to work. This is mentioned in their documentation: https://plotly.com/python/getting-started/#jupyterlab-support

Author: vinegar-tom

episodes/data-visualisation.md

@@ -146,6 +146,8 @@ plotting package in Python called Plotly. First let’s install and then use the
 ```python
 # uncomment below to install plotly if the import fails. 
 # !pip install plotly
+# you may also need to install ipywidgets.
+# !pip install ipywidgets
 import plotly.express as px
 ```