Change Public Key Auth with SSH link

I have changed the Public Key Authentication with SSH from http://library.linode.com/security/ssh-keys to https://www.linode.com/docs/networking/ssh/use-public-key-authentication-with-ssh/
Update using-the-linode-shell-lish.md

Updated the proposed link change to a relative link, which helps us when we work on guides locally.
Merge pull request #1 from alexfornuto/patch-1

Update using-the-linode-shell-lish.md
Merge pull request #132 from Traestar/master

Change Public Key Auth with SSH link