Fix link to badge in README

[alexdewar]

Description

Accidentally broke this by changing the name of the workflow in the last PR. Let's fix it, otherwise we'll get failures from the link checker.

Type of change

• Bug fix (non-breaking change to fix an issue)
• New feature (non-breaking change to add functionality)
• Refactoring (non-breaking, non-functional change to improve maintainability)
• Optimization (non-breaking change to speed up the code)
• Breaking change (whatever its nature)
• Documentation (improve or add documentation)

Key checklist

• All tests pass: $ cargo test
• The documentation builds and looks OK: $ cargo doc

Further checks

• Code is commented, particularly in hard-to-understand areas
• Tests added that prove fix is effective or that feature works

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.34%. Comparing base (9456a6b) to head (6130641).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #619   +/-   ##
=======================================
  Coverage   85.34%   85.34%           
=======================================
  Files          38       38           
  Lines        3399     3399           
  Branches     3399     3399           
=======================================
  Hits         2901     2901           
  Misses        305      305           
  Partials      193      193           

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