Reword description for rebase level

On first pass, I interpretted this phrasing to mean that some change must be made to the master branch, when in fact it is the feature branch that needs to change its base from 'init commit' to 'add content'.

This new suggested wording attempts to make it a bit clearer that the user must operate on the feature branch, and is semantically consistent with `--onto` option described in the man page for git rebase.