secret-handshake: fix broken link in README.md

exercises/secret-handshake/README.md

 
 ## Source
 
-Bert, in Mary Poppins [http://www.imdb.com/character/ch0011238/quotes](http://www.imdb.com/character/ch0011238/quotes)
+Bert, in Mary Poppins [http://www.imdb.com/title/tt0058331/quotes/qt0437047](http://www.imdb.com/title/tt0058331/quotes/qt0437047)
 
 ## Submitting Incomplete Solutions