completion: clarify installation instruction for zsh

completion: clarify installation instruction for zsh

The original comment does not describe type of ~/.zsh/_git explicitly
and zsh does not warn or fail if a user create it as a dictionary.
So unexperienced users could be misled by the original comment.

There is a small update to clarify it.

Helped-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Maxim Belsky <public.belsky@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>