Correct 'McpTool' to 'McpServerTool' in README (#775)

Author: asklar

README.md

@@ -88,7 +88,7 @@ var response = await chatClient.GetResponseAsync(
 Here is an example of how to create an MCP server and register all tools from the current application.
 It includes a simple echo tool as an example (this is included in the same file here for easy of copy and paste, but it needn't be in the same file...
 the employed overload of `WithTools` examines the current assembly for classes with the `McpServerToolType` attribute, and registers all methods with the
-`McpTool` attribute as tools.)
+`McpServerTool` attribute as tools.)
 
 ```
 dotnet add package ModelContextProtocol --prerelease