Fix badges in README

[jhdark]

Proposed changes

Describe the big picture of your changes here to communicate to the maintainers why we should accept this pull request. If it fixes a bug or resolves a feature request, be sure to link to that issue.

Types of changes

What types of changes does your code introduce to FESTIM?

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Code refactoring
• Documentation Update (if none of the other choices apply)
• New tests

Checklist

• Black formatted
• Unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

Further comments

If this is a relatively large or complex change, kick off the discussion by explaining why you chose the solution you did and what alternatives you considered, etc...

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.87%. Comparing base (19a899f) to head (994c9bc).
⚠️ Report is 4 commits behind head on fenicsx.

Additional details and impacted files

@@           Coverage Diff            @@
##           fenicsx    #1050   +/-   ##
========================================
  Coverage    94.87%   94.87%           
========================================
  Files           44       44           
  Lines         3100     3100           
========================================
  Hits          2941     2941           
  Misses         159      159           

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