Correct latex typo for exponentiated kernel (#223)

Small typo : missing `\left)` for the exponentiated kernel

Author: theogf

docs/src/kernels.md

@@ -75,7 +75,7 @@ where $\gamma > 0$.
 The [`ExponentiatedKernel`](@ref) is defined as
 
 ```math
-  k(x,x') = \exp\left(\langle x,x'\rangle).
+  k(x,x') = \exp\left(\langle x,x'\rangle\right).
 ```
 
 ## Fractional Brownian Motion