Typo in docs of the Cosine Kernel

[tirthasheshpatel]

https://github.com/pymc-devs/pymc3/blob/3b765a52dbae3772ae72ee7f16b446a2eaee7547/pymc3/gp/cov.py#L470

This line should be k(x, x') = \mathrm{cos}\left( 2 \pi \frac{||x - x'||}{ \ell^2} \right)

Reference: https://gpflow.readthedocs.io/en/master/gpflow/kernels/#gpflow-kernels-cosine