Change output of `shelly backup list`

I changed header in` shelly backup list` output, instead of:

![12bfe846-9206-11e2-95ff-eabfeeb5e66e](https://f.cloud.github.com/assets/127219/284945/01c3d6b8-9208-11e2-9a8f-f21ba2b92198.png)

display this:

![198d9ff6-9206-11e2-8f91-576c1c8c1970](https://f.cloud.github.com/assets/127219/284946/0631532e-9208-11e2-90dd-eac919192185.png)