Correct test's usage of non-square nested arrays

This resolves an apparent discrepancy between the 'it' description and the actual test code. Another option would be to just change the 'it' description but since the comments in `session3/challenge/5_blocks.rb` specify that `spiral_access` takes 2d array that is square, I assume the tests should reflect this usage.

If, on the other hand, you'd like `spiral_access` to handle all 2d arrays (e.g. other rectangles), perhaps there should be a couple tests to make that explicit.

P.S. Thank you so much for creating Ruby Kickstart. It's been a while since I've gone through it much, but when I was starting to learn Ruby, it helped me sooooo much!