slt-release-finish: use the same tag in all places

/to @sam-github or @rmg please review
I found out earlier when I saw @sam-github using the `${1:?version is mandatory}` syntax and checked if it was POSIX. Turns out there are a few substitutions available under POSIX: http://pubs.opengroup.org/onlinepubs/007904975/utilities/xcu_chap02.html#tag_02_06_02