Update calling_a_callback_function_more_than_once.js

Great resource, thanks!  Ran into a small error within one of the arguments passed to the bothArraysAreEqual function.  Figured I would just fix it rather than create an issue.  Hope that's ok.
Merge pull request #11 from treiff/patch-1

Fix typo in calling_a_callback_function_more_than_once.js