Reword SSH private key for CD docs

Tiny thing I had lying around - since I stumbled over the wording the last time.

As always, please only merge if you think this is an improvement.

morph/grid/local/private-keys/README.rst

@@ -11,7 +11,7 @@ You can find more information about some of these secrets in ``ops/generating-ke
 deploy_key
 ----------
 
-This is an SSH private key which will be authorized to trigger a deployment update on the deployment hosts themselves.
+This SSH private key authenticates CD and authorizes triggering a deployment update on the deployment hosts.
 The corresponding SSH public key is kept in the ``public-keys`` location.
 
 grafana-admin.password