tuned signature: fewer warnings in IntelliJ IDEA;

--- a/src/Pure/General/sql.scala	Wed Mar 13 10:58:15 2024 +0100
+++ b/src/Pure/General/sql.scala	Wed Mar 13 11:04:06 2024 +0100
@@ -406,7 +406,7 @@
   /* notifications: IPC via database server */
 
   sealed case class Notification(channel: String, payload: String = "") {
-    override def toString =
+    override def toString: String =
       "Notification(" + channel + if_proper(payload, "," + payload) + ")"
   }