outdated term "master" in "Working with a sky-bag" code

[Wither-Bane]

outdated terms in "Working with a sky-bag",
the repos created since October-2020 now have "main" as their default branch instead of the previous "master" branch. changing the instructions to "git push -u origin main " instead of "git push -u origin master" will help new users complete this step without extended research :)