Fix zsh install location (#532)

Reported: NixOS/nixpkgs#222667 (comment)

Author: Freed-Wu

Makefile

@@ -12,7 +12,7 @@ prefix = /usr/local/
 includedir = $(prefix)/include
 libdir = $(prefix)/lib
 sysconfdir = $(prefix)/etc
-zshcompletiondir = $(prefix)/share/zsh/site_functions
+zshcompletiondir = $(prefix)/share/zsh/site-functions
 
 OBJS = src/common.o src/main.o