Update obsolete receiver comment in C# 14 extension block sample

docs/csharp/programming-guide/classes-and-structs/snippets/ExtensionMembers/CustomExtensionMembers.cs

@@ -117,7 +117,7 @@
     public uint id;
     public float balance;
 
     private int secret;
 }
 
 //<UserDefinedRef>
@@ -125,7 +125,8 @@
 {
     extension(ref Account account)
     {
-        // ref keyword can also appear before the this keyword
+        // account is the receiver (analogous to the this parameter in extension methods).
+        // The ref modifier on the extension block means the receiver is passed by reference.
         public void Deposit(float amount)
         {
             account.balance += amount;