Yuck, that's a poor excuse for a sinewave.
Both the circuits I posted make a better sinewave than that.
The simple RC filter shaper gives a reasonable sinewave.
**broken link removed**
Here's what the waveform looks like with the component values given above:
See, it isn't perfect but it's better than the kit.
It can be improved by increasing C3 but the gain needs to be increased (increase R5) which will reduce the maximum usable frequency.
As you seem to be pretty new to this, I'd recommend going for the basic RC filter wave shaper.
Here's a site which gives the formula to calculate the frequency of the triangle wave generator I used in my circuit:
Here's another site with a more detailed explanation:
Op Amp Triangle-Wave Generator
C2 is a DC blocking capacitor so it needs to be high enough to pass the lowest frequency of interest.
The minimum value can be calculated using the following formula:
[latex]C = \frac{1}{RF}[/latex]
R is the value of the load impedance, in this case R1 + R4
F is the frequency when the signal will be reduced to 70.7% of the maximum value, so it's probably a good idea to set this variable to halve the minimum frequency of interest.
C is the required capacitance in Farads.