Update README.md

fix link to Assignment 3 in Core Systems

Author: Maizbest

README.md

@@ -221,7 +221,7 @@ Courses | Duration | Effort | Additional Text / Assignments| Prerequisites
 [Introduction to Computer Science - CS50](https://www.edx.org/course/introduction-computer-science-harvardx-cs50x#!) ([alt](https://cs50.harvard.edu/)) | 12 weeks | 10-20 hours/week | After the sections on C, skip to the next course. [Why?](FAQ.md#why-do-you-recommend-skipping-the-second-half-of-cs50) | introductory programming
 [Build a Modern Computer from First Principles: From Nand to Tetris](https://www.coursera.org/learn/build-a-computer) ([alt](http://www.nand2tetris.org/)) | 6 weeks | 7-13 hours/week | - | C-like programming language
 [Build a Modern Computer from First Principles: Nand to Tetris Part II ](https://www.coursera.org/learn/nand2tetris2) | 6 weeks | 12-18 hours/week | - | one of [these programming languages](https://user-images.githubusercontent.com/2046800/35426340-f6ce6358-026a-11e8-8bbb-4e95ac36b1d7.png), From Nand to Tetris Part I
-[Introduction to Computer Networking](https://lagunita.stanford.edu/courses/Engineering/Networking-SP/SelfPaced/about)| 8 weeks | 4–12 hours/week | [Assignment 1](https://github.com/PrincetonUniversity/COS461-Public/tree/master/assignments/assignment1)<br>[Assignment 2](https://www.scs.stanford.edu/10au-cs144/lab/reliable/reliable.html)<br>[Assignment 3](https://nptel.ac.in/courses/106105080/pdf/M2L7.pdf)<br>[Assignment 4](http://www-net.cs.umass.edu/wireshark-labs/Wireshark_TCP_v7.0.pdf) | algebra, probability, basic CS
+[Introduction to Computer Networking](https://lagunita.stanford.edu/courses/Engineering/Networking-SP/SelfPaced/about)| 8 weeks | 4–12 hours/week | [Assignment 1](https://github.com/PrincetonUniversity/COS461-Public/tree/master/assignments/assignment1)<br>[Assignment 2](https://www.scs.stanford.edu/10au-cs144/lab/reliable/reliable.html)<br>[Assignment 3](https://nptel.ac.in/content/storage2/courses/106105080/pdf/M2L7.pdf)<br>[Assignment 4](http://www-net.cs.umass.edu/wireshark-labs/Wireshark_TCP_v7.0.pdf) | algebra, probability, basic CS
 [ops-class.org - Hack the Kernel](https://www.ops-class.org/) | 15 weeks | 6 hours/week | Replace course textbook with [Operating Systems: Three Easy Pieces](http://pages.cs.wisc.edu/~remzi/OSTEP/) | algorithms
 
 ### Core theory