build: Fix installation of zsh completions in user-specified directory

This copy/paste from bash could end up installing into "$(DESTDIR)yes",
which is not as intended.

Signed-off-by: Simon McVittie <smcv@collabora.com>

configure.ac

@@ -62,7 +62,7 @@ AC_ARG_WITH([zsh-completion-dir],
 if test "x$with_zsh_completion_dir" = "xyes"; then
         [ZSH_COMPLETION_DIR="$datadir/zsh/site-functions"]
 else
-    ZSH_COMPLETION_DIR="$with_bash_completion_dir"
+    ZSH_COMPLETION_DIR="$with_zsh_completion_dir"
 fi