Fix docs link in tutorials/spectrum_models.ipynb
"The following clases will be used:\n",
"\n",
"* [gammapy.spectrum.models.PowerLaw](http://docs.gammapy.org/dev/api/gammapy.spectrum.models.PowerLaw.html)\n",
- "* [gammapy.utils.modelling.Parameter](http://docs.gammapy.org/dev/api/gammapy.utils.modeling.Parameter.html)\n",
- "* [gammapy.utils.modelling.Parameters](http://docs.gammapy.org/dev/api/gammapy.utils.modeling.Parameters.html)\n",
+ "* [gammapy.utils.fitting.Parameter](http://docs.gammapy.org/dev/api/gammapy.utils.fitting.Parameter.html)\n",
+ "* [gammapy.utils.fitting.Parameters](http://docs.gammapy.org/dev/api/gammapy.utils.fitting.Parameters.html)\n",
"* [gammapy.spectrum.models.SpectralModel](http://docs.gammapy.org/dev/api/gammapy.spectrum.models.SpectralModel.html)"
]
},
