s/purge/prune/g

It isn't purge, it's prune. I think I got confused with the Docker terminology.

README.md

@@ -10,7 +10,7 @@ It's more or less the equivalent of
 
 ```bash
 git checkout main
-git fetch --all --purge  # purge is to delete unnecessary remote refs
+git fetch --all --prune  # prune is to delete unnecessary remote refs
 git merge origin/main main --ff-only
 git branch --merged | grep -v main | xargs git branch -d
 ```