C#: Add example of sealing ToString on a record type.

Author: michaelnebel

csharp/ql/test/library-tests/csharp10/RecordTypeSealedToString.cs

@@ -0,0 +1,6 @@
+using System;
+
+public record MyEntry(string Name, string Address)
+{
+    sealed public override string ToString() => $"{Name} lives at {Address}";
+};