completion: zsh: fix name due to broken autoloading

completion: zsh: fix name due to broken autoloading

Commit 176f5adfdb wrongly changed the installation path to
'~/.zsh/git-completion.zsh', this ensures the script is not
automatically loaded.

The whole point of adding the script to the fpath variable is that it's
autoloaded after typing 'git<tab>', which won't happen unless it's named
_git.

I've changed the wording so it's crystal clear the name of the file
*must* be '_git'.

http://zsh.sourceforge.net/Doc/Release/Completion-System.html#Autoloaded-files

Cc: Maxim Belsky <redacted>
Cc: Johannes Schindelin <redacted>
Signed-off-by: Felipe Contreras <redacted>
Signed-off-by: Junio C Hamano <redacted>