Reword ArgumentError message

Change 'ArgumentError: wrong number of arguments (%d for %d)' to ArgumentError: wrong number of arguments (%d instead of %d).

This clarifies the meaning of the message without any room for doubt.

And looks like raises many doubts: http://stackoverflow.com/questions/7537450/what-does-wrong-number-of-arguments-1-for-0-mean-in-ruby

Personally, I did find the wording weird the first time I saw that error message and had to google it to make sure I got it right. With this pull request, we leave no room for further doubt.