Remove stale reference to WhiteNoiseKernel
"This specification leaves a large amount of flexibility when defining a model. For example, to compose two kernels via addition, you can either add the kernel modules directly:\n",
"\n",
"```python\n",
- "self.covar_module = ScaleKernel(RBFKernel() + WhiteNoiseKernel())\n",
+ "self.covar_module = ScaleKernel(RBFKernel() + LinearKernel())\n",
"```\n",
"\n",
"Or you can add the outputs of the kernel in the forward method:\n",
