update colab instructions

Author: ianhi

docs/installing.md

@@ -29,7 +29,12 @@ TODO
 
 ### Google Colab
 
-To use `ipympl` in colab follow the instructions [here](https://colab.research.google.com/github/googlecolab/colab-cdn-widget-manager/blob/main/notebooks/ipympl.ipynb)
+To use `ipympl` in colab run these lines:
+
+```python
+from google.colab import output
+output.enable_custom_widget_manager()
+```
 
 ## Mixing Frontend and Backend Versions