Switch 3b1b Linear Algebra from pre to corequisite (#927)

Switch 3b1b Essence of Linear Algebra from prerequisite of MIT Linear Algebra to corequisite

README.md

@@ -382,7 +382,7 @@ Courses | Duration | Effort | Prerequisites
 Courses | Duration | Effort | Prerequisites | Discussion
 :-- | :--: | :--: | :--: | :--:
 [Essence of Linear Algebra](https://www.youtube.com/playlist?list=PLZHQObOWTQDPD3MizzM2xVFitgF8hE_ab) | - | - | [high school math](FAQ.md#how-can-i-review-the-math-prerequisites) | [chat](https://discord.gg/m6wHbP6)
-[Linear Algebra](https://ocw.mit.edu/courses/mathematics/18-06sc-linear-algebra-fall-2011/) | 14 weeks | 12 hours/week | Essence of Linear Algebra | [chat](https://discord.gg/k7nSWJH)
+[Linear Algebra](https://ocw.mit.edu/courses/mathematics/18-06sc-linear-algebra-fall-2011/) | 14 weeks | 12 hours/week | corequisite: Essence of Linear Algebra | [chat](https://discord.gg/k7nSWJH)
 [Introduction to Logic](https://www.coursera.org/learn/logic-introduction) | 10 weeks | 4-8 hours/week | [set theory](https://www.youtube.com/playlist?list=PL5KkMZvBpo5AH_5GpxMiryJT6Dkj32H6N) | [chat](https://discord.gg/MbM2Gg5)
 [Probability](https://projects.iq.harvard.edu/stat110/home) | 24 weeks | 12 hours/week | [Differentiation and Integration](https://www.edx.org/course/calculus-1b-integration) | [chat](https://discord.gg/UVjs9BU)