fix: Don't complete _command on zsh (#1690)

Previously the generated zsh completion script started with the line

    #compdef _<command> <command>

where <command> is the command that the zsh completion script is
generated for.

This enabled completions for both <command> and _<command>, but
_<command> is the completion function itself and should not be
completed. Furthermore, attempting to autocomplete _<command> (e.g.
typing "_<command><Space><Tab>" in a zsh shell) causes zsh to hang.

This commit fixes the #compdef line to only complete <command>, not
_<command>.

Co-authored-by: Arvid Norlander <VorpalBlade@users.noreply.github.com>