Fix Typo in docs

[kentongray]

Context

Typo in docs

How did you test your changes?

Doc change, NA

Types of changes

• Breaking change (a fix or a feature because of which an existing functionality doesn't work as expected anymore)
• New feature or improvement (a non-breaking change that adds functionality)
• Bug fix (a non-breaking change that fixes an issue)
• Additional language file, or a change to an existing language file (translations)
• [ X] Change to the documentation

Related issues:

NA

Checklist:

• [ X] I have reviewed the guidelines about Contributing to HyperFormula and I confirm that my code follows the code style of this project.
• [X ] I have signed the Contributor License Agreement.
• [ X] My change is compliant with the OpenDocument standard.
• [ X] My change is compatible with Microsoft Excel.
• [ X] My change is compatible with Google Sheets.
• I described my changes in the CHANGELOG.md file.
• [ X] My changes require a documentation update.
• My changes require a migration guide.