remove https and use http in the link (#124)

Author: mmcky

lectures/intro.md

@@ -28,5 +28,5 @@ For an overview of the series, see [this page](https://quantecon.org/python-lect
 
 
 ```{admonition} Previous website
-While this new site will receive all future updates, you may still view the [old site here](https://rst-python.quantecon.org) for the next month.
+While this new site will receive all future updates, you may still view the [old site here](http://rst-python.quantecon.org) for the next month.
 ```