Update Cov and Means GP notebook

[canyon289]

Notebook title:Mean and Covariance Functions Notebook url: https://docs.pymc.io/en/v3/pymc-examples/examples/gaussian_processes/GP-MeansAndCovs.html

Issue description

The images require scrolling. Fixing up so they don't

Expected output

Figures that don't require scrolling

Proposed solution

Change Figure size