It will never be a proper triangle wave, because you are measuring the charging of a capacitor through a resistor. That is what the link shows.
What component values did you use? There are great 555 simulators on the Internet, as well as LT Spice. After all of these years, it is unlikely the 555 is not working properly. The most likely explanations are that you are getting exactly what you should be getting for the values you used, or you wired it up incorrectly. That is why seeing the waveform you are getting and knowing the component values you used is important.
John