Update Old Khan Academy Resources to College Prep School for Prereqs

[DanyPell]

The "high school math" prereqs should link to the appropriate College Prep School links on YouTube, just like the OSSU-Computer Science has updated them to.

[pulkitkrishna00]

#55 is open since more than 7 months ago

But unless the owner of OSSU gets the time to merge it, it is going to stay this way.