fix mining help (#6885)

was
        --min-gas-price=[STRING]
	                Minimum amount of Wei per GAS to be paid for a
			transaction to be accepted for mining. Overrides
			--basic-tx-usd.

Issue: --basic-tx-usd does not exist. Author probably meant
--usd-per-tx.

New:
        --min-gas-price=[STRING]
	                Minimum amount of Wei per GAS to be paid for a
			transaction to be accepted for mining. Overrides
			--usd-per-tx.