Fix typo in Value Retrievers doc: change Registr to Register

[ronaldbosma]

Fixed a typo in the Value Retrievers documentation page. Changed Registr to Register. Fixes issue #2544

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 not work as expected).
• [ ] Performance improvement
• [ ] Refactoring (so no functional change)
• [X] Other (docs, build config, etc)

Checklist:

• [ ] I've added tests for my code. (most of the time mandatory)
• [ ] I have added an entry to the changelog. (mandatory)
• [ ] My change requires a change to the documentation.
• [X] I have updated the documentation accordingly.

[SabotageAndi]

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