test(aliby): fix argument logic in test_image

WHY IS THIS CHANGE NEEDED?:
- https://git.ecdf.ed.ac.uk/swain-lab/aliby/aliby/-/jobs/56762 failed
  because TypeError: compute() takes 1 positional argument but 2 were given

HOW DOES THE CHANGE SOLVE THE PROBLEM?:
- corrected a stupid typo: compute() is a function that takes no
  arguments that gives out an array, and i want to index that array

REFERENCES:
- issue #53