fix psfmap definition
@@ -26,4 +26,4 @@ The remaining IRF classes implement:
- `~gammapy.irf.EnergyDependentMultiGaussPSF` a PSF whose probability density is parametrised by the sum of 1 to 3 2-dimensional gaussian (definition at :ref:`gadf:psf_3gauss`);
- `~gammapy.irf.PSFKing` a PSF whose probability density is parametrised by the King function (definition at :ref:`gadf:psf_king`);
- `~gammapy.irf.PSFKernel` a PSF that can be used to convolve `~gammapy.maps.WcsNDMap` objects;
-- `~gammapy.irf.PSFMap` a PSF that can be used to convolve `~gammapy.maps.WcsNDMap` objects;
+- `~gammapy.irf.PSFMap` a four-dimensional `~gammapy.maps.Map` storing a `~gammapy.irf.PSFKernel` for each sky position.
