Fix warning in doc building for SpectrumSimulation removed class
"We will use the following classes:\n",
"\n",
"* [gammapy.spectrum.SpectrumDatasetOnOff](https://docs.gammapy.org/dev/api/gammapy.spectrum.SpectrumDatasetOnOff.html)\n",
- "* [gammapy.spectrum.SpectrumSimulation](https://docs.gammapy.org/dev/api/gammapy.spectrum.SpectrumSimulation.html)\n",
+ "* [gammapy.spectrum.SpectrumEvaluator](https://docs.gammapy.org/dev/api/gammapy.spectrum.SpectrumEvaluator.html)\n",
+ "* [gammapy.spectrum.SpectrumDataset](https://docs.gammapy.org/dev/api/gammapy.spectrum.SpectrumDataset.html)\n",
"* [gammapy.irf.load_cta_irfs](https://docs.gammapy.org/dev/api/gammapy.irf.load_cta_irfs.html)\n",
"* [gammapy.spectrum.models.PowerLaw](https://docs.gammapy.org/dev/api/gammapy.spectrum.models.PowerLaw.html)"
]
