Update walletdb comment after renaming.
Update walletdb comment after renaming.

Text from https://github.com/bitcoin/bitcoin/pull/11851#issuecomment-350320608
by John Newbery <john@johnnewbery.com>.

thelazier/dash,UdjinM6/dash,thelazier/dash,UdjinM6/dash,thelazier/dash,dashpay/dash,dashpay/dash,thelazier/dash,thelazier/dash,dashpay/dash,UdjinM6/dash,dashpay/dash,UdjinM6/dash,UdjinM6/dash,dashpay/dash