luci-app-aria2: fix the log option

According to the package aria2, the log option
is enable_logging, so fix this option name.

As luci-app-aria2 can't set custom log file dir,
the default log file is /var/log/aria2.log but not
in the config file dir.

Signed-off-by: Zheng Qian <sotux82@gmail.com>