Show example of plotting vector parameter in vignette

[jgabry]

Submission Checklist

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

Summary

Add example plot of a vector parameter to posterior.Rmd vignette

closes #443

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):
Jonah Gabry

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

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.87%. Comparing base (d83f62d) to head (0d54735).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1223      +/-   ##
==========================================
+ Coverage   91.79%   91.87%   +0.08%     
==========================================
  Files          15       15              
  Lines        6200     6241      +41     
==========================================
+ Hits         5691     5734      +43     
+ Misses        509      507       -2     

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[jgabry]

@avehtari I requested a review since this is closing an issue you opened