PB API response key consistency

[programmable-banking-community]

Just some inconsistencies I've picked up:

1. Account & transactions have their data contents in data.accounts/data.transactions, but beneficiary categories does not have an equivalent categories array.
2. PB docs have incorrect key casing for the get beneficiaries call, and the beneficiary categories doc has outdated key names (e.g. id instead of CategoryId).
3. Key casing on beneficiary categories is PascalCase instead of camelCase as used everywhere else.

Reported by: Nikolaos Spyratos