added hyperlink to video series

README.md

@@ -1273,7 +1273,7 @@ You'll get more graph practice in Skiena's book (see Books section below) and th
     - [ ] Twitter:
         - [O'Reilly MySQL CE 2011: Jeremy Cole, "Big and Small Data at @Twitter" (video)](https://www.youtube.com/watch?v=5cKTP36HVgI)
         - [Timelines at Scale](https://www.infoq.com/presentations/Twitter-Timeline-Scalability)
-    - For even more, see "Mining Massive Datasets" video series in the Video Series section.
+    - For even more, see "Mining Massive Datasets" video series in the [Video Series](#video-series) section.
 - [ ] Practicing the system design process: Here are some ideas to try working through on paper, each with some documentation on how it was handled in the real world:
     - review: [The System Design Primer](https://github.com/donnemartin/system-design-primer)
     - [System Design from HiredInTech](http://www.hiredintech.com/system-design/)