Fix wrong variable name (tmux-plugins#20)

Author: mrymtsk

prefix_highlight.tmux

@@ -13,7 +13,7 @@ output_suffix='@prefix_highlight_output_suffix'
 show_copy_config='@prefix_highlight_show_copy_mode'
 show_sync_config='@prefix_highlight_show_sync_mode'
 copy_attr_config='@prefix_highlight_copy_mode_attr'
-sync_attr_config='@prefix_highlight_sync_attr'
+sync_attr_config='@prefix_highlight_sync_mode_attr'
 prefix_prompt='@prefix_highlight_prefix_prompt'
 copy_prompt='@prefix_highlight_copy_prompt'
 sync_prompt='@prefix_highlight_sync_prompt'