Fix typo in dask best practice/cape docs

[lbdreyer]

🚀 Pull Request

Description

I noticed a small typo in one of the examples in the dask best practice section: https://scitools-iris.readthedocs.io/en/v3.7.1/further_topics/dask_best_practices/dask_parallel_loop.html#original-code-with-loop

I have not checked the rest of the code works, but I thought it would be worth getting this typo fixed at least

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Consult Iris pull request check list

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.80%. Comparing base (ea1e2dd) to head (90a2792).
Report is 77 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6380   +/-   ##
=======================================
  Coverage   89.80%   89.80%           
=======================================
  Files          90       90           
  Lines       23752    23752           
  Branches     4418     4418           
=======================================
  Hits        21331    21331           
  Misses       1672     1672           
  Partials      749      749           

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