* Fix machine learning in core applications

The machine learning course is only 3 weeks long, not 11. Further, it's a very gentle introductory course. Even the prerequisite of Basic coding is stretching it, but it is as listed on the course page.

* Fix machine learning 

Machine learning should link to the entire specialization. The specialization is 11 weeks in all I believe, and they suggest 9 hours per week although that could be scaled down to 4-6 hours if you're just auditing. Also, the prerequisite should be basic coding, linear algebra is not necessary.

Closes #1118