Fix inconsistent test case number in comments and console logs (dotnet#10367)

Test 13's comment incorrectly mentioned "test 11" instead of "test 12." The node (dog) was removed in test 12, not in test 11. This could lead to confusion for future developers reading the code.

Author: depth233

snippets/csharp/System.Collections.Generic/LinkedListT/Overview/source.cs

@@ -102,7 +102,7 @@ public static void Main()
 
         // Add the node after the node referred to by mark2.
         sentence.AddAfter(mark2, current);
-        IndicateNode(current, "Test 13: Add node removed in test 11 after a referenced node (brown):");
+        IndicateNode(current, "Test 13: Add node removed in test 12 after a referenced node (brown):");
 
         // The Remove method finds and removes the
         // first node that that has the specified value.
@@ -231,7 +231,7 @@ private static void IndicateNode(LinkedListNode<string> node, string test)
 //Test 12: Remove current node (dog) and attempt to indicate it:
 //Node 'dog' is not in the list.
 
-//Test 13: Add node removed in test 11 after a referenced node (brown):
+//Test 13: Add node removed in test 12 after a referenced node (brown):
 //the quick brown (dog) jumps over the lazy old fox
 
 //Test 14: Remove node that has the value 'old':