Fix typo in Backend API specification
MODIFY
WalletWasabi.Backend/README.md
WalletWasabi.Backend/README.md
Requests have the following format: `/api/v3/{coin}/{controller}/`.
Currently supported coins: `btc`.
- For example requesting fees: `GET /api/v3/btc/offchain/exchange-rates`.
+ For example requesting exchange rate: `GET /api/v3/btc/offchain/exchange-rates`.
### Controller: Blockchain, Coin: BTC

Fix typo in Backend API specification