micro:bit accelerometer shake - event number

[epeach]

Between versions of firmware, the values representing the accelerometer changed: microbit-foundation/microbit-firmata#12 (comment)
Updating our reference to the accelerometer to account for this change.

PR Checklist:

• Tests provide adequate coverage
• Privacy and Security impacts have been assessed
• Code is well-commented
• New features are translatable or updates will not break translations
• Relevant documentation has been added or updated
• User impact is well-understood and desirable
• Pull Request is labeled appropriately
• Follow-up work items (including potential tech debt) are tracked and linked