Configuration settings indicate incorrect access

[jtheuerkauf]

https://github.com/LucioFranco/jj-spr/blob/cb583737af712ba6829d5066a3767ef9c287d691/README.md?plain=1#L174-L187

This section of the README states spr's config is stored in git, but it's actually in jj:

$ git config spr.branchPrefix
[nothing]
$ jj config get spr.branchPrefix
spr/jtheuerkauf/