fix in-sample deviance

[aloctavodia]

Fix #1288

When computing the "in-sample deviance" we were adding the wrong factor, except for the "deviance" scale.

• Follows official PR format
• Code style correct (follows pylint and black guidelines)
• Changes are listed in changelog

[codecov]

Codecov Report

Merging #1435 into master will decrease coverage by 0.07%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master    #1435      +/-   ##
==========================================
- Coverage   91.99%   91.91%   -0.08%     
==========================================
  Files         105      105              
  Lines       11184    11200      +16     
==========================================
+ Hits        10289    10295       +6     
- Misses        895      905      +10     

Impacted Files	Coverage Δ
arviz/plots/backends/bokeh/compareplot.py	91.07% <50.00%> (-8.93%)	⬇️
arviz/plots/backends/matplotlib/compareplot.py	86.95% <50.00%> (-10.42%)	⬇️

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