Fix documentation of CosineKernel (#231)

Author: devmotion

docs/src/kernels.md

@@ -40,11 +40,9 @@ The [`ZeroKernel`](@ref) is defined as
 The [`CosineKernel`](@ref) is defined as
 
 ```math
-  k(x, x') = \cos(\pi |x-x'|),
+  k(x, x') = \cos(\pi \|x-x'\|).
 ```
 
-where $x\in\mathbb{R}$.
-
 ## Exponential Kernels
 
 ### Exponential Kernel