Use spherical trigonometry in PSF formulae

[jknodlseder]

The formula

.. math:: \int_{0}^{\infty} 2 \pi r dP/dr(r) dr = 1, where dP/dr = 2 \pi r dP/d\Omega

on the PSF page is only true for small angles, the correct formula is

.. math:: \int_{0}^{\infty} 2 \pi sin(r) dP/dr(r) dr = 1, where dP/dr = 2 \pi sin(r) dP/d\Omega

Also in the description above the small angle approximation is used. The exact formulae should be given as for some instruments, the maximum PSF radius can become quite large, and the difference will be perceptible.

[cdeil]

Fine with me to put the more precise formulae.

Can you please make a pull request?

For such little changes you can just go to the file on Github and click the pen symbol in the top right to edit the file and make a pull request directly: https://github.com/open-gamma-ray-astro/gamma-astro-data-formats/blob/master/source/irfs/psf/index.rst