Add space before plugins.etc_dir in emqx.conf

etc/emqx.conf

@@ -1839,7 +1839,7 @@ module.rewrite = off
 ## The etc dir for plugins' config.
 ##
 ## Value: Folder
-plugins.etc_dir ={{ platform_etc_dir }}/plugins/
+plugins.etc_dir = {{ platform_etc_dir }}/plugins/
 
 ## The file to store loaded plugin names.
 ##