docs: fix code coverage badge

[jelledouwe]

Description

Motivation and Context

• I have raised an issue to propose this change (required for new features and bug fixes)

Types of changes

• Bug fix (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 change)
• Documentation (update in the documentation)

Checklist:

• I have read the CONTRIBUTION guide (required)
• I have used semantic commit messages such that my PR is semantic (required)
• I have updated the changelog accordingly (required).
• My change requires a change to the documentation.
• I have updated the tests accordingly (required for a bug fix or a new feature).
• I have updated the documentation accordingly.
• I have reformatted the code using make codestyle (required)
• I have checked the codestyle using make check-codestyle and make lint (required)
• I have ensured make pytest passes. (required)
• I have checked that the documentation builds using make doc (required)

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