fix(release): fix typo in ff-master option, should use underscore

[hanks]

ff-master should be ff_master in the code, or else can not read the option value correctly in code below:

if git_remote_branch_exists "$ORIGIN/$MASTER_BRANCH"; then
    if flag ff_master; then

[ChrisJStone]

I'm in the process of updating gitflow. If you could kindly resubmit your pull request against my fork here as it appears this fork is no longer being updated.