examples/basicauth: Simplify comma-ok type assertion in if statement. (google#558)

According to https://golang.org/ref/spec#Type_assertions:

> For an expression x of interface type and a type T, the primary expression
>
> 	x.(T)
>
> asserts that x is not nil and that the value stored in x is of type T. [...] The
> value of ok is true if the assertion holds.

Therefore, this if statement can be simplified without changing behavior.

Author: dmitshur

examples/basicauth/main.go

@@ -37,7 +37,7 @@ func main() {
 	user, _, err := client.Users.Get("")
 
 	// Is this a two-factor auth error? If so, prompt for OTP and try again.
-	if _, ok := err.(*github.TwoFactorAuthError); err != nil && ok {
+	if _, ok := err.(*github.TwoFactorAuthError); ok {
 		fmt.Print("\nGitHub OTP: ")
 		otp, _ := r.ReadString('\n')
 		tp.OTP = strings.TrimSpace(otp)