change hint about session4 to above 8 rather than above 20. fixes #7

TopDog.tex

@@ -281,7 +281,7 @@ Below are the required assignments (there are hidden hints!):
         from the slides.}
     \item[Session Management Challenge 4]
         {\color{white}Can you guess a Session ID\@? It should
-        be somewhat larger than 20.}
+        be somewhat larger than 8.}
     \item[Failure to Restrict URL Access 2]
     \item[Cross Site Scripting 3]
     \item[Insecure Cryptographic Storage Challenge 3]