please update description information re citations

[HedvigS]

first off, love the package!

currently, the information that is pulled for the package for citation is wrong. The paper with the title Using stacking to average Bayesian predictive distributions was published in 2018, not 2017

> citation("loo")
To cite the loo R package:

  Vehtari A, Gabry J, Magnusson M, Yao Y, Bürkner P, Paananen T, Gelman A (2024). “loo: Efficient leave-one-out
  cross-validation and WAIC for Bayesian models.” R package version 2.8.0, <https://mc-stan.org/loo/>.

To cite the loo paper:

  Vehtari A, Gelman A, Gabry J (2017). “Practical Bayesian model evaluation using leave-one-out cross-validation and WAIC.”
  _Statistics and Computing_, *27*, 1413-1432. doi:10.1007/s11222-016-9696-4 <https://doi.org/10.1007/s11222-016-9696-4>.

To cite the stacking paper:

  Yao Y, Vehtari A, Simpson D, Gelman A (2017). “Using stacking to average Bayesian predictive distributions.” _Bayesian
  Analysis_. doi:10.1214/17-BA1091 <https://doi.org/10.1214/17-BA1091>.

To see these entries in BibTeX format, use 'print(<citation>, bibtex=TRUE)', 'toBibtex(.)', or set
'options(citation.bibtex.max=999)'.

[jgabry]

Thank you for pointing out the incorrect year in the citation for the stacking paper. I just opened #284 to fix it.