fix multi rope for forest plot

[aloctavodia]

Fix #1382

• Follows official PR format
• Includes a sample plot to visually illustrate the changes (only for plot-related functions)
• New features are properly documented (with an example if appropriate)?
• Includes new or updated tests to cover the new feature
• Code style correct (follows pylint and black guidelines)
• Changes are listed in changelog

[codecov]

Codecov Report

Merging #1390 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1390   +/-   ##
=======================================
  Coverage   91.72%   91.73%           
=======================================
  Files         105      105           
  Lines       10901    10909    +8     
=======================================
+ Hits         9999    10007    +8     
  Misses        902      902           

Impacted Files	Coverage Δ
arviz/plots/forestplot.py	91.17% <ø> (ø)
arviz/plots/backends/bokeh/forestplot.py	94.68% <100.00%> (+0.06%)	⬆️
arviz/plots/backends/matplotlib/forestplot.py	98.03% <100.00%> (+0.02%)	⬆️

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