Update plotting_backends.md with correct path for function

[Haleshot]

Change path of function provided in documentation to right path.

Description

Fixed incorrect path provided for the get_plotting_function() provided in documentation as arviz.plots.get_plotting_function to correct path - arviz.plots.plot_utils.get_plotting_function

Checklist

• 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

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📚 Documentation preview 📚: https://arviz--2364.org.readthedocs.build/en/2364/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.01%. Comparing base (1fd6ad8) to head (4ca32cb).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2364   +/-   ##
=======================================
  Coverage   87.01%   87.01%           
=======================================
  Files         124      124           
  Lines       12788    12788           
=======================================
  Hits        11128    11128           
  Misses       1660     1660           

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[OriolAbril]

Good catch, thanks!