[Bug] Typo in doc - Matern Kernel
# 🐛 Bug

Should the documentation here https://docs.gpytorch.ai/en/v1.1.1/kernels.html#maternkernel have $ \\Theta^{-2}$ in the definition of `d` like the other kernels (example RBF)? 

diff --git a/gpytorch/kernels/matern_kernel.py b/gpytorch/kernels/matern_kernel.py
index f8b2793f..4701b092 100644
--- a/gpytorch/kernels/matern_kernel.py
+++ b/gpytorch/kernels/matern_kernel.py
@@ -23,7 +23,7 @@ class MaternKernel(Kernel):
 
     where
 
-    * :math:`d = (\\mathbf{x_1} - \\mathbf{x_2})^\\top \\Theta^{-1} (\\mathbf{x_1} - \\mathbf{x_2})`
+    * :math:`d = (\\mathbf{x_1} - \\mathbf{x_2})^\\top \\Theta^{-2} (\\mathbf{x_1} - \\mathbf{x_2})`
       is the distance between
       :math:`x_1` and :math:`x_2` scaled by the :attr:`lengthscale` parameter :math:`\\Theta`.
     * :math:`\\nu` is a smoothness parameter (takes values 1/2, 3/2, or 5/2). Smaller values are less smooth.
diff --git a/gpytorch/settings.py b/gpytorch/settings.py
index cb720054..81359462 100644
--- a/gpytorch/settings.py
+++ b/gpytorch/settings.py
@@ -471,7 +471,7 @@ class max_preconditioner_size(_value_context):
     """
     The maximum size of preconditioner to use. 0 corresponds to turning
     preconditioning off. When enabled, usually a value of around ~10 works fairly well.
-    Default: 0
+    Default: 15
     """
 
     _global_value = 15
['gpytorch/settings.py', 'gpytorch/kernels/matern_kernel.py']