Update posterior vignette

[jgabry]

Submission Checklist

• Run unit tests
• Declare copyright holder and agree to license (see below)

Summary

Small additions to the posterior.Rmd vignette

Copyright and Licensing

Please list the copyright holder for the work you are submitting
(this will be you or your assignee, such as a university or company):
Columbia University

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/)

[codecov-commenter]

Codecov Report

Merging #797 (020c812) into master (5b3426f) will decrease coverage by 1.72%.
The diff coverage is n/a.

❗ Current head 020c812 differs from pull request most recent head 0bd325e. Consider uploading reports for the commit 0bd325e to get more accurate results

@@            Coverage Diff             @@
##           master     #797      +/-   ##
==========================================
- Coverage   88.27%   86.56%   -1.72%     
==========================================
  Files          12       12              
  Lines        4152     4152              
==========================================
- Hits         3665     3594      -71     
- Misses        487      558      +71     

see 5 files with indirect coverage changes

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[jgabry]

Going to just go ahead and merge this small (doc only) PR