Update instructions on granting backup host access to GHES (#959)

docs/getting-started.md

@@ -31,8 +31,8 @@
     * In a clustering environment, the `GHE_EXTRA_SSH_OPTS` key must be configured
       with the `-i <abs path to private key>` SSH option.
 
- 3. Add the backup host's SSH key to the GitHub appliance as an *Authorized SSH
-    key*. See [Adding your SSH key to the ssh-agent][4] for instructions.
+ 3. Add the backup host's SSH public key to the GitHub Enterprise Server appliance, in order to grant it administrative shell access.
+    See [Accessing the GitHub Enterprise Server administrative shell (SSH)][4] for instructions.
 
  4. Run `bin/ghe-host-check` to verify SSH connectivity with the GitHub
     appliance.
@@ -42,4 +42,4 @@
 [1]: https://github.com/github/backup-utils/releases
 [2]: https://github.com/github/backup-utils/releases/tag/v2.11.4
 [3]: https://github.com/github/enterprise-backup-site/blob/master/backup.config-example
-[4]: https://docs.github.com/enterprise-server/authentication/connecting-to-github-with-ssh/generating-a-new-ssh-key-and-adding-it-to-the-ssh-agent#adding-your-ssh-key-to-the-ssh-agent
+[4]: https://docs.github.com/enterprise-server/admin/configuration/configuring-your-enterprise/accessing-the-administrative-shell-ssh