Update obsolete receiver comment in C# 14 extension block sample

[Copilot]

The extension block sample on the extension members article used an outdated comment that referenced this, even though the sample uses extension(ref Account account). This update aligns the comment with current extension block receiver semantics.

• What changed

  • Updated the comment in docs/csharp/programming-guide/classes-and-structs/snippets/ExtensionMembers/CustomExtensionMembers.cs to describe account as the receiver, analogous to the legacy this parameter.
  • Clarified that ref on the receiver means the receiver is passed by reference.

• Snippet updated

  extension(ref Account account)
  {
      // account is the receiver (analogous to the this parameter in extension methods), and ref means the receiver is passed by reference
      public void Deposit(float amount)
      {
          account.balance += amount;
      }
  }

• Fixes Obselete comment #54847

[Copilot]

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[BillWagner]

This is ready for final review.