Edit README sample code error in dotnet

[Masumi-M]

Motivation and Context

1. Why is this change required?
   => There was a bug in README file, and it may confuse for beginners.
2. What problem does it solve?
   => The sample code in dotnet code.
3. What scenario does it contribute to?
   => When the beginner try to execute code using README.
4. If it fixes an open issue, please link to the issue here.
   => README Sample Code Error in dotnet #1037

Description

The following code are changed.

kernel.Config.AddOpenAITextCompletionService("davinci-openai",
    "text-davinci-003",               // OpenAI Model name
    "...your OpenAI API Key..."       // OpenAI API Key
);

kernel.Config.AddOpenAITextCompletionService(
    "text-davinci-003",               // OpenAI Model name
    "...your OpenAI API Key..."       // OpenAI API Key
);

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows SK Contribution Guidelines (https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md)
• The code follows the .NET coding conventions (https://learn.microsoft.com/dotnet/csharp/fundamentals/coding-style/coding-conventions) verified with dotnet format
• All unit tests pass, and I have added new tests where possible
• I didn't break anyone 😄