alzheimer_test gave the wrong answer

[mhaber]

Task was to find a T out of a 10 x 30 matrix filled with I. The T was in row 10, column 3 but the test said it is in 10, 4.

[yihui]

Sounds like someone else has reported this to me in private before, but I don't have the time to fix it. There is certainly an off-by-one error somewhere. If you can find and fix it, I'll be happy to merge your pull request.