docs: update artifact jobspec sshkey example path. (#17077)

website/content/docs/job-specification/artifact.mdx

@@ -141,7 +141,7 @@ artifact {
     # Make sure that the system known hosts file is populated:
     # ssh-keyscan github.com | sudo tee -a /etc/ssh/ssh_known_hosts
     # https://github.com/hashicorp/go-getter/issues/55
-    sshkey = "${base64encode(file("/etc/ssh/ssh_known_hosts"))}"
+    sshkey = "${base64encode(file("/path/to/private-key"))}"
   }
 }
 ```