Fix non-working alias gSu

I'm guessing the current alias `alias gSu='git submodule foreach git pull origin master'` is from the early days of git? The command doesn't work for me, but my proposed change to `alias gSu='git submodule update'` does. It's also possible I've misinterpreted what `gSu` is supposed to do and am usin git wrong (hopefully not!)