luci-app-aria2: fix incorrect "Extra Settings" section option name

The "Extra Settings" section `extra_setting` option name does not match what
the `net/aria2` package expects.

See:

- https://github.com/openwrt/packages/blob/openwrt-19.07/net/aria2/files/aria2.init#L311
- https://github.com/openwrt/packages/blob/openwrt-19.07/net/aria2/files/aria2.conf#L31

Causing the luci UI "Extra Settings" option to have no effect. This fix
renames `extra_setting` => `extra_settings`.

Signed-off-by: Raymond Wanyoike <raymond.wanyoike@gmail.com>