Closed Issue: Fix tracing info link to fwlink in webapiconfig comment [840]

// To disable tracing in your application, please comment out or remove the following line of code
// For more information, refer to: http://www.asp.net/web-api
config.EnableSystemDiagnosticsTracing();

The current code points to http://www.asp.net/web-api. We should change it to fwlink and point it to tracing content.
Comments: The tracing api is moved out from WebApiConfig.cs file and the link in the readme.txt is correct.