add --global to listing the git settings

[radersma]

While teaching the git lesson (Setting Up Git) we noticed that the following command resulted in some confusion:

git config --list

For Windows users this command shows many more system settings than for Linux or Mac users, which made people wondering whether there was something wrong with their settings. Perhaps it is better to specify to only show the global settings:

git config --list --global

[bencomp]

We noticed this too. Specifically, some users' settings had init.defaultbranch=master (system config) and init.defaultbranch=main (our --global config). I haven't looked to find out if the latter overrides the former, i.e. if it mattered in practice, but it did raise a little confusion.