Update borovitskiy21a to include erratum.

[aterenin]

This PR corrects a minor error in one of the tables of borovitskiy21a, "Matérn Gaussian Processes on Graphs".

The error is in Table 1. The presented values were incorrectly listed as MSE, instead of RMSE, and were incorrectly multiplied by the constant 1/\sqrt{75}. This error affects both the proposed method and baselines in the same way, and therefore does not affect relative comparisons between methods or validity of the paper's core results.

This version corrects this and adds the following text to the final main-content page. Adding this text does not result change the total number of pages used by the work.

No other parts of the paper, beyond Table 1, are changed. The supplementary material for the work is also unchanged.

\section*{Erratum}

In the original published manuscript, Table~\ref{table:mat_diff_comp} incorrectly labeled RMSE values as MSE.
In addition, the RMSE values presented were incorrectly scaled by a constant, and were equal to $\frac{1}{\sqrt{75}}\f{RMSE}$ instead of $\f{RMSE}$.
This scaling difference was applied to both the proposed method and baselines, and does not affect relative comparisons between methods.
The current manuscript version corrects both aforementioned errors.

[aterenin]

Tagging @vabor112 and @lawrennd

[lawrennd]

Thanks Alex. For these errata, the decision to accept is with the proceedings editors. Can you confirm you have permission to update from Arindam Banerjee and/or Kenji Fukumizu