Fix broken retina background-image for footer learn-links

The battery icon is missing in the footer on retina:

![screen shot 2013-12-14 at 12 38 12 pm](https://f.cloud.github.com/assets/1831398/1749204/501908d0-6500-11e3-9669-6aa25ef87362.png)
![screen shot 2013-12-14 at 12 37 45 pm](https://f.cloud.github.com/assets/1831398/1749205/501913e8-6500-11e3-9b34-e672f0be8170.png)

Fixed the background-image to render from proper folder:
`doc/assets/scss/_footer.scss`

![screen shot 2013-12-14 at 12 38 43 pm](https://f.cloud.github.com/assets/1831398/1749206/5029e54c-6500-11e3-877c-51ba89e144ea.png)
![screen shot 2013-12-14 at 12 38 52 pm](https://f.cloud.github.com/assets/1831398/1749207/502ab30a-6500-11e3-9215-31966cf23157.png)