Update basic-auth.md - fix example

[AahilRafiq]

• reflect the updated function signature of Basic auth validator

Before

e.Use(middleware.BasicAuth(func(username, password string, c *echo.Context) (bool, error) {
	// Be careful to use constant time comparison to prevent timing attacks
	if subtle.ConstantTimeCompare([]byte(username), []byte("joe")) == 1 &&
		subtle.ConstantTimeCompare([]byte(password), []byte("secret")) == 1 {
		return true, nil
	}
	return false, nil
}))

After

e.Use(middleware.BasicAuth(func(c *echo.Context, username, password string) (bool, error) {
	// Be careful to use constant time comparison to prevent timing attacks
	if subtle.ConstantTimeCompare([]byte(username), []byte("joe")) == 1 &&
		subtle.ConstantTimeCompare([]byte(password), []byte("secret")) == 1 {
		return true, nil
	}
	return false, nil
}))