Mention SSH key repository access in 'Restoring Files with Borg' page

[nfraprado]

Hi, I was following Restoring Files with Borg to restore a backup on a new computer and it took me quite a while to figure out that not only an SSH key needed to be added to BorgBase, but that I had to assign it to specific repository under the Access tab.

This information is specific to BorgBase, so I think it is one of the most important pieces of information to be mentioned in that page.