Update README.rst

[lewisblake]

update doi

Change Summary

update DOI v0.21.0

Related issue number

Checklist

• Start with a draft-PR
• The PR title is a good summary of the changes
• PR is set to AeroTools and a tentative milestone
• Documentation reflects the changes where applicable
• Tests for the changes exist where applicable
• Tests pass locally
• Tests pass on CI
• At least 1 reviewer is selected
• Make PR ready to review

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.00%. Comparing base (90ddba7) to head (b4b3158).
Report is 2 commits behind head on main-dev.

Additional details and impacted files

@@            Coverage Diff            @@
##           main-dev    #1308   +/-   ##
=========================================
  Coverage     79.00%   79.00%           
=========================================
  Files           136      136           
  Lines         20797    20797           
=========================================
  Hits          16430    16430           
  Misses         4367     4367           

Flag	Coverage Δ
unittests	79.00% <ø> (ø)

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