Correct brackets in preprocessor documentation for list of seasons

[bouweandela]

Description

The brackets used in the documentation of the seasons argument to several preprocessor functions were not a valid way to create a list in YAML. This pull request fixes the problem.

Link to documentation: https://esmvaltool--1420.org.readthedocs.build/projects/ESMValCore/en/1420/recipe/preprocessor.html#seasonal-statistics

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[codecov]

Codecov Report

Merging #1420 (595e721) into main (ffa16c4) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #1420   +/-   ##
=======================================
  Coverage   89.34%   89.34%           
=======================================
  Files         196      196           
  Lines       10234    10234           
=======================================
  Hits         9144     9144           
  Misses       1090     1090           

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[valeriupredoi]

HNY @bouweandela - cheers for this very short and biutiful PR (I really didn't want to start the new year with a 4,000-line PR 🤣 )