.Net: Update GettingStarted Documentation (#10453)

### Motivation and Context

Please help reviewers and future users, providing the following
information:
1. Why is this change required? Documentation is incorrect and a
struggle for someone getting started
2. What problem does it solve? Someone new to the codebase would be
confused by the documentation
3. What scenario does it contribute to? Someone new to the codebase
would be confused by the documentation

### Description

ChatPromptSync does not exist.

Author: lastcoolnameleft

dotnet/samples/Concepts/README.md

@@ -13,7 +13,7 @@ dotnet test -l "console;verbosity=detailed" --filter "FullyQualifiedName=NameSpa
 Example for `ChatCompletion/OpenAI_ChatCompletion.cs` file, targeting the `ChatPromptSync` test:
 
 ```powershell
-dotnet test -l "console;verbosity=detailed" --filter "FullyQualifiedName=ChatCompletion.OpenAI_ChatCompletion.ChatPromptSync"
+dotnet test -l "console;verbosity=detailed" --filter "FullyQualifiedName=ChatCompletion.OpenAI_ChatCompletion.ChatPromptAsync"
 ```
 
 ## Table of Contents