Update importprivkey named args documentation
Summary:
Fixes #11462. Updated documentation for importprivkey function to use the correct name for the first argument. Also updates a call to importprivkey to use named args in functional test.
Backport of core PR11465
https://github.com/bitcoin/bitcoin/pull/11465/files
Test Plan:
./src/bitcoin-cli help importprivkey
Check the help message is correct
./test/functional/test_runner.py wallet_importprunedfunds
Reviewers: #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Differential Revision: https://reviews.bitcoinabc.org/D3411