generatetoaddress should mention ABC wallet not Core

Summary:
The help for generatetoaddress mentions "core wallet". Change it to
mention "ABC wallet" instead.

Test Plan:

ninja
src/bitcoind
src/bitcoin-cli help generatetoaddress

Verify changes to help text

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, majcosta

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D6617