Invalid source links in History slides

[shannonh800]

In the "History" slides from last week, the links for "History of Open Source" and "The 9 most important events in open source history" take you to invalid pages (give 404 errors). These are on slide 17 and can be found in the github here on lines 215 and 219, respectively.

[Jiajun-Jiao]

I'd like to claim this issue, the correct link to "The 9 most important events in open source history" should be https://www.pingdom.com/blog/the-9-most-important-events-in-open-source-history/. However, for "History of Open Source" I noticed that the learning center (https://www.longsight.com/learning-center) for longsight is down, so we cannot access its original website.

[joannakl]

@Jiajun-Jiao You can work on this if you still want to. Just remove the link that is broken.