Readme Update SetLogFile

[AnthonyPoschen]

// Create a client instance
c, err := paypalsdk.NewClient("clientID", "secretID", paypalsdk.APIBaseSandBox)
c.SetLogFile("/tpm/paypal-debug.log") // Set log file if necessary

is now incorrect after my pull request.

possible solution
// Create a client instance
c, err := paypalsdk.NewClient("clientID", "secretID", paypalsdk.APIBaseSandBox)
c.SetLog(os.Stdout) // Set log to terminal stdout