completion: fix zsh installation instructions

- Fix wrong script in completion configuration. zsh wants bash completion
  path here, not path to itself.

- Add `compinit` autoload command, since whole thing didn't work
  if it is not loaded.

Signed-off-by: Alexey <lesha.ogonkov@gmail.com>
Reviewed-by: Stefan Haller <lists@haller-berlin.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.zsh

@@ -9,13 +9,14 @@
 #
 # If your script is somewhere else, you can configure it on your ~/.zshrc:
 #
-#  zstyle ':completion:*:*:git:*' script ~/.git-completion.zsh
+#  zstyle ':completion:*:*:git:*' script ~/.git-completion.bash
 #
 # The recommended way to install this script is to make a copy of it in
 # ~/.zsh/ directory as ~/.zsh/git-completion.zsh and then add the following
 # to your ~/.zshrc file:
 #
 #  fpath=(~/.zsh $fpath)
+#  autoload -Uz compinit && compinit
 
 complete ()
 {