graph of transfer function for RC filter
The graph is correct. The curve looks a bit unusual because the voltage scale is linear. The Bode plot will have a slope of 6dB/octave which will be a straight line on a logarithmic graph. But a straight line on a logarithmic graph is a hyperbola on a semi-logarithmic graph.
If the curve were to approach a decreasing asymptote the voltage would eventually become negative. Not something you get from a divider.