file links are wrong

[tklimk]

There are some file links do not work, like:
https://lhcb.github.io/second-analysis-steps/code/building-decays/01.historical.py
mentioned in
https://lhcb.github.io/second-analysis-steps/building-decays-part1.html.
And
https://lhcb.github.io/second-analysis-steps/code/building-decays/02.optimized.py
mentioned in
https://lhcb.github.io/second-analysis-steps/building-decays-part2.html