[RPC] fix help texts
[RPC] fix help texts

- getblockindexstats
- getmintsinblocks
- getserials

Mrs-X/Darknet,Mrs-X/Darknet,PIVX-Project/PIVX,npccoin/npccoin,Mrs-X/Darknet,Mrs-X/PIVX,npccoin/npccoin,npccoin/npccoin,Darknet-Crypto/Darknet,PIVX-Project/PIVX,Mrs-X/Darknet,Mrs-X/PIVX,PIVX-Project/PIVX,Mrs-X/PIVX,PIVX-Project/PIVX,PIVX-Project/PIVX,npccoin/npccoin,Mrs-X/PIVX,npccoin/npccoin,PIVX-Project/PIVX,Mrs-X/Darknet,Darknet-Crypto/Darknet,PIVX-Project/PIVX,Mrs-X/PIVX,Darknet-Crypto/Darknet,Mrs-X/PIVX,Darknet-Crypto/Darknet,npccoin/npccoin,PIVX-Project/PIVX,Darknet-Crypto/Darknet,PIVX-Project/PIVX,Mrs-X/Darknet,Darknet-Crypto/Darknet