Update PAPL URL to latest version (#2934)

free-programming-books.md

@@ -740,7 +740,7 @@ Kerridge (PDF) (email address *requested*, not required)
 * [Models of Computation](http://cs.brown.edu/people/jes/book/) - John E. Savage
 * [Practical Foundations for Programming Languages, Preview](https://www.cs.cmu.edu/~rwh/pfpl/2nded.pdf) - Robert Harper (PDF)
 * [Principles of Programming Languages](https://web.archive.org/web/20150418034451/http://www.cs.jhu.edu/~scott/pl/book/dist/) - Scott F. Smith
-* [Programming and Programming Languages](http://papl.cs.brown.edu/2014/index.html) - Shriram Krishnamurthi
+* [Programming and Programming Languages](https://papl.cs.brown.edu/2016/) - Shriram Krishnamurthi
 * [Programming in Martin-Löf's Type Theory](http://www.cse.chalmers.se/research/group/logic/book/) - Bengt Nordstroem
 * [Programming Languages: Application and Interpretation (2nd Edition)](http://cs.brown.edu/~sk/Publications/Books/ProgLangs/)
 * [Semantics with Applications: A Formal Introduction](http://www.daimi.au.dk/~bra8130/Wiley_book/wiley.html) - Hanne Riis Nielson, Flemming Nielson