Fix IList.Insert example to handle index == Count case (#12285)

Author: Copilot

snippets/csharp/System.Collections/IList/Overview/Program.cs

@@ -103,7 +103,7 @@ public int IndexOf(object value)
 
     public void Insert(int index, object value)
     {
-        if ((_count + 1 <= _contents.Length) && (index < Count) && (index >= 0))
+        if ((_count + 1 <= _contents.Length) && (index <= Count) && (index >= 0))
         {
             _count++;
 

snippets/visualbasic/System.Collections/IList/Overview/Program.vb

@@ -90,7 +90,7 @@ Public Class SimpleList
 
     Public Sub Insert(ByVal index As Integer, ByVal value As Object) Implements IList.Insert
 
-        If _count + 1 <= _contents.Length AndAlso index < Count AndAlso index >= 0 Then
+        If _count + 1 <= _contents.Length AndAlso index <= Count AndAlso index >= 0 Then
             _count += 1
 
             For i As Integer = Count - 1 To index Step -1