reference doc Changes

[CharmisG]

Resolved below points :

1. CRITICAL -- This line now throws an error if tipSettings, an optional field, is undefined.
2. This name doesn't match the name in the documentation, Money.
3. amount should be required per documentation.
4. currencyCode should be optional per documentation.

[hukid]

• [x] please address this comment: https://github.com/square/react-native-square-reader-sdk/pull/148#discussion_r987183078