Correct link to Pong

The [current link](http://elm-lang.org/misc/Pong.html) to Pong at the end of the blog post just returns this error:

> Parse error at (line 1, column 1):
unexpected end of input
expecting reserved word 'module', reserved word 'import' or at least one datatype or variable definition

This commit changes the link target to [this page](http://elm-lang.org/blog/games-in-elm/part-0/Pong.html).