Typos in Exponential and Mattern 1/2 covariance functions

[perrette]

Mean and Covariance Functions:
https://www.pymc.io/projects/examples/en/latest/gaussian_processes/GP-MeansAndCovs.html#exponential:

Issue description

There is a mistake in the Exponential and Mattern 1/2 covariance functions:

• Exponential: the parameter l is not squared (it should be homogeneous to the distance)
• Mattern 1/2: the distance should appear, not the square of the distance