Fix README typo

Issues

N/A

Proposed changes

Simple typo fix

Checklist

• [X] I consent that this change becomes part of alphaTab under it's current or any future open source license
• [X] Changes are implemented
• [X] Existing builds tests pass
• [ ] New tests were added

Further details

• [ ] This is a breaking change
• [ ] This change will require update of the documentation/website