docs: name the matrix host options by their new path

Every prose and error-message reference to the moved options, including
two in hive-c0re/src/matrix.rs that omitted the services. prefix and so
read as the per-agent option. An error that names an option the operator
cannot find is worse than no hint.

hivectl/src/open.rs

@@ -30,7 +30,7 @@ pub(crate) async fn open_url(socket: &Path, target: OpenTarget) -> Result<()> {
         ),
         OpenTarget::Matrix => (
             urls.matrix,
-            "the matrix GUI URL needs `services.hyperhive.matrix.gui.enable = true`",
+            "the matrix GUI URL needs `services.hyperhive.swarm.matrix.gui.enable = true`",
         ),
     };
     let url = url.with_context(|| format!("no URL available for this surface — {hint}"))?;