fixed redirect_url accessor

[rgaudin]

lib.cxx mirrors libzim's getRedirectUrl() with get_redirect_url().
That methods had incorrectly been named redirect_url.

python-libzim/libzim/lib.cxx

Lines 158 to 164 in 3092492

	zim::writer::Url
	ZimArticleWrapper::getRedirectUrl() const
	{
	std::string redirectUrl = callCythonReturnString("get_redirect_url");
	return zim::writer::Url(redirectUrl.substr(0, 1)[0], redirectUrl.substr(2, redirectUrl.length()));
	}