[fix] use 'path' variable for '-a' option use explanation for 'ynh app install' cmd.
[fix] use 'path' variable for '-a' option use explanation for 'ynh app install' cmd.

YunoHost/moulinette-yunohost,YunoHost/moulinette-yunohost,YunoHost/moulinette-yunohost,YunoHost/moulinette-yunohost,YunoHost/yunohost,YunoHost/yunohost,YunoHost/yunohost,YunoHost/moulinette-yunohost,YunoHost/yunohost