Fix mode on contrib/completion/zsh/_docker (-x)

ZSH completion scripts aren't supposed to be executable - they're sourced by ZSH and aren't run directly.

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #6427 from tianon/fix-zsh-completion-mode

Fix mode on contrib/completion/zsh/_docker (-x)