Make output match code (#47618)

Fix a small typo reported via UUF

Author: BillWagner

docs/csharp/language-reference/operators/snippets/shared/AssignmentOperator.cs

@@ -9,7 +9,7 @@ public static void Examples()
         // <Usage>
         string msg = "Hi";
         RefReassignAndModify(ref msg);
-        Console.WriteLine(msg); // Output: Hi!
+        Console.WriteLine(msg); // Output: Hi
         // </Usage>
     }