There is no problem with your function, however you only calculated eleven data points (check the length of x), which gives a very triangular plot because it cannot represent the detail of the curve that you want. You need to increase how many points you calculate, such as this which generates one hundred and one data points:
x = -0.5:0.01:0.5; % <- changed the step to 0.01
y = (400/pi)*cos(x*pi).^2.*sinc(20*sin(x*pi)).^2;
plot(x,y);
