Add missing preprocessor name rolling_window_statistics to example.

[katjaweigel]

Description

In the documentation the preprocessor name "rolling_window_statistics" is missing in the example. Therefore copying the example in a recipe leads to an error.

Closes #2891

Link to documentation: https://esmvaltool--2890.org.readthedocs.build/projects/ESMValCore/en/2890/recipe/preprocessor.html#rolling-window-statistics

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.63%. Comparing base (f17b767) to head (c533761).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2890   +/-   ##
=======================================
  Coverage   95.63%   95.63%           
=======================================
  Files         263      263           
  Lines       15492    15492           
=======================================
  Hits        14816    14816           
  Misses        676      676           

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