[Docs] Incorrect documentation for `MultitaskMultivariateNormal` distribution

[SaiAakash]

📚 Documentation/Examples

The documentation for the MultitaskMultivariateNormal distribution says it has a covar attribute which is incorrect. This distribution only has an attribute called covariance_matrix and not covar.

I can add a PR to fix this in the docstring of the MultitaskMultivariateNormal class.

[gpleiss]

If you could put up a PR with a fix, that'd be great!