fix rdname for ess_mean

[n-kall]

Summary

ess_mean.default was pointing to the ess_quantile documentation page, this fixes it to point to the ess_mean page

Copyright and Licensing

By submitting this pull request, the copyright holder is agreeing to license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
• Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

[paul-buerkner]

thanks!