Copyright statement

[haesleinhuepf]

Please go through the slides and add copyright statements if they are missing, e.g. on the XKCD slides here:
https://github.com/BiAPoL/Quantitative_Bio_Image_Analysis_with_Python_2022/blob/main/docs/day3c_statistics_and_tests/Statistics.pdf

[jo-mueller]

Should be fixed in #54