[Customers Product]: Change references from 'Users' to 'Customers' within documentation 👥

[sumitramanga]

[Customers Product]: Change references from 'User tracking' to 'Customers' 👤

NOTE: This is not to be merged until we have fully released the name change in the Raygun application. Changes to 'users' within the code needs to be discussed and planned for.

Description 📝

This PR is to updating the name convention for 'User tracking' to now be 'Customers' to reflect what's within the Raygun application.

Updates 👉 Update README documentation 👉 Update sample app comment to refer to Customers

Test plan 🧪

• Make sure documentation changes are reflected to be 'Customers' instead of 'User Tracking'

Checklist ✔️

• [ ] Builds pass
• [ ] Reviewed by another developer
• [ ] Code is written to standards