Ananta Waranie

Fixed inaccurate durations for [Calculus](https://www.edx.org/xseries/mitx-18.01x-single-variable-calculus) and [Differential Equations](https://www.edx.org/xseries/mitx-18.03x-differential-equations) [courses](https://github.com/ossu/math#calculus). Also, is there a reason why [this](https://openlearninglibrary.mit.edu/courses/course-v1:OCW+6.042J+2T2019/about) wasn't chosen instead for the [Discrete Math](https://github.com/ossu/math#discrete-mathematics) course?

Perbaiki typo tanda penyingkat.