completion: zsh: fix bash script extension

completion: zsh: fix bash script extension

Commit 0e5ed7cca3 wrongly changed the extension of the bash script
to .zsh; the zstyle configuration is for the slave script (bash), not
the master one (zsh).

For example it could be:

  zstyle ':completion:*:*:git:*' script ~/.git-completion.bash

The extension doesn't really matter, but it confuses people into
thinking it's a zsh script; it's not.

Cc: Peter van der Does <peter@avirtualhome.com>
Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>